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Saitama's power is reversing cause and effect on attack and defense.
Like Lancer's Gae Bolg from Fate/stay night, he does something; and reality warps so that the effect is that his punch crushes people, or their attacks on him aren't as effective; regardless of his physical strength. Similar to Lancer; the ability has an Equivalent Exchange on his luck. Unlike Lancer, he doesn't get hurt; however people never recognize what he does. (Because they can't rationalize the shift in cause and effect.)

Saitama's punches are a result of Newton's third law of motion not affecting him.
Alternative to the above, why Saitama's strength is so immense is that he has the ability to actively (or passively) negate the third law of motion, so when he punches something, there is no equal and opposite reaction that is repelling said punch.

Saitama is a product of the same phenomenon as the random mysterious beings that are mutated human beings.
The series has introduced examples of human beings that have repetitively performed certain tasks that cause them to transform in a fitting fashion. A man who ate too many crabs became Crabrante, a giant crab man. A person shadowboxed a ceiling light pull cord so much that he became Personification of a Light Pull Cord, a powerful and fast boxing giant. These beings were still people, as they spoke normally and the first example even engaged in a somewhat civil conversation with Saitama at first passing. They were influenced by their power, however. One sought revenge for being drawn upon, and the second desired to test his strength against objects and people.

While these beings were people influenced by their transformations to the extent that they became disruptive to the lives of others, Saitama is different from them only in that he is heroic and doesn't desire to harm innocent people with his power. His extremely powerful punch and generally superhuman fitness is a showing of his desire for strength that he sought through training. He trained so much and so hard that whatever phenomenon caused the changes in the other two beings mentioned happened to him as well. As the others transformed into monsters, Saitama transformed as well; he simply became bald. Thus, he is literally a monster fighting other monsters, but he is a monster that retains most of his original human form.

  • A similar transformation occurred in King of Thorn where a tranformation Mass Super-Empowering Event turned a girl into the shape and form of her own twin sister.
  • confirmed by Word of God in an author's note somewhere that I can't find.
  • Maybe that's why Licenseless Rider is able to survive basically any fight he gets into, despite always losing. His obsession is doing good, so the phenomenon turned him into someone who would always be there to help people as best he can. He didn't become a monster or suffer any kind of ill effects (like baldness) because he has an absolutely noble goal, rather than one for personal gain (Saitama is a "hero for fun", whereas Mumen Rider honest-to-God just wants to help people).

Speed of Sound Sonic and Flashy Flash are related.

Both of them have redundant names, boast their superior speed and use the Wind Blade Kick as well as sharing extremely similar combat styles. And both reacted exactly the same when meeting Saitama for the first time.


One-Punch Man takes place in the future of Mob Psycho 100.
Mob psycho despite officially being part of the same world, lacks all the mysterious beings and there are no heroes, and psychics and ghosts are only slowly becoming a thing (also a young Tatsumaki and Fubuki are shown at one point). Most likely something happened between Mob Psycho and One-Punch Man that led to a mass empowering, allowing people to push the limits of humanity or go over the edge and become monsters. Mob Psycho likely takes place around 8-10 years before One Punch Man, and BLAST is widely believed to be a grown up Mob (who would be around the same age as Tatsumaki, + or - a couple years).
  • Alternatively, BLAST is Ritsu, Mob's brother who was always more outgoing than Mob and seems more likely to have gone "hero" than the Mob whose main desire is to be normal and pretend he has no powers.
    • THAT COULD WORK OUT A LOT. Mob could've been the source of the Mysterious Beings as well and Ritsu became a hero to help deal with the aftermath, feeling partially responsible as his brother. If ???% somehow takes over Mob, his powers could go all wonky and cause these supernatural beings and powers to emerge. Or it could have been the after effects of the divine tree/broccoli. Still, Mob could easily be Blast, though. After the manga, he helps others just like Reigen does (sort of) by using his powers regularly now. If so, Blast's speech to Tatsumaki and his lack of publicity makes sense since the manga is pretty much Mob's growth as a person and his want to be just "Mob", despite his powers.

Saitama will eventually find a challenging enemy... Who will beat the living shit out of him because he's too strong.
Because honestly, the irony would be too great to see.
  • For Bonus Points, this will be during his absolute Darkest Hour-The Big Bad has defeated all the other heroes, Genos is severely injured/dying (again), Saitama ignores the Big Bad's bragging and punches him to oblivion-and fails.
    • For extra Bonus Points, the bad guy's strength will come from a slightly better version of Saitama's training regimen: 110 push-ups, 110 sit-ups, 110 squats, and running 10.5 km every single day, or something like that.
      • For extra, extra Bonus Points, it will be Blast

By the time Genos finds the cyborg that destroyed his village, he will vastly outclass him
Think about it: Genos has been shown to ask Stench to improve him every time they meet up. Genos has also be shown to fight on even terms with city destroying monsters (e.g. Sea King), and power level far below that would be enough to destroy a village. Therefore, it seems likely that by the time Genos finds the mad cyborg, he will be far above him in terms of strength.
  • For bonus points, Genos will destroy the mad cyborg with one punch.

Related to the above theory: Drive Knight is the cyborg that Genos is looking for
Drive Knight tells Genos to watch out for Metal Knight, who Drive Knight probably also has a beef with. Drive Knight will wait till Genos and Metal Knight fight and wound each other before finishing them off.

The series will end with Saitama fighting God
It will play out similar Asura's Wrath: God reveals that he's responsible for much of the Earth's suffering (or maybe is just too lazy to do anything about it), Saitama gets pissed, they fight (with Saitama finally fighting at 100%) and Saitama either A) killing God, B) dying in the attempt, or C) both.

Metal Knight and the Hero Association are secretly part of an organization creating the Mysterious Beings
Metal Knight is a jerkass from the start as he uses a situation of a city-destroying asteroid to test out some new weapons without actually saving anyone. His actions later on include avoiding combat, looting alien technologies, and building a super-fortress for the Hero Association. That also makes the Hero Association suspect as without the Monsters there would be no need for Heroes or for people paying the Heroes. What do they need with a Super-fortress anyways? There is already evidence that the Hero Association is supplanting city organizations like the police with Heroes. the Hero Association itself seems full of weak and/or jerkass people with a few strong and/or real heroes so they're nearly useless against any threat unless Saitama saves them.

The Hero Association is going to become a dictatorship.
Let's see. The H.A. is going out of its way to make the police look like fools and useless fodder. The Heroes are slowly becoming the first and last line of defense with no military units or government organizations in sight. Metal Knight is producing bigger and badder weapons "for peace". They built a superfortess on the ruins of a devastated city that no civilian is allowed access to instead of making it a super bunker for resident civilians. Metal Knight has a walker ready made to make the fortress in 7 days as if he KNEW that the city would be destroyed. The H.A. seems more concerned with public appearances and naming superheroes instead of protecting the people. The great prophet conveniently dies right before giving the details to the upcoming catastrophe causing confusion and panic while under the direct watch of the H.A. who should have been monitoring her the entire time. Mysterious Beings have started popping up everywhere with seemingly no explanation and people with super powers are inexplicably popping up everywhere as well. Plus Drive Knight tells Genos that Metal Knight is his enemy. Something fishy is going on.

King actually does have a superpower: His luck
He may not be the most badass superhero alive, but he's been in so many situations that could've splatted him, and yet managed to survive from several timely factors, and even gotten a terrifying reputation he can save his own life with more than once, it all seems to be a little too much for a coincidence. So perhaps something within him is organizing these coincidences, letting him leech off others' fame for himself. He and his luck don't seem to agree too much on what they want, but it'd seem that's his actual power.

This world is the future (or past) of Soul Eater
Just like the Kishin, madness and obsession can gift people with Lovecraftian superpowers. The only difference is that heroes have relatively benign obsessions while Mysterious Beings succumb to the madness inherent in theirs. In the more powerful heroes like Tatsumaki, Metal Knight and Fubuki we start seeing the genesis of Witches. Saitama might be a precursor to (or actually IS) Shinigami, someone who has effectively become a Humanoid Abomination en route to ascending to an Anthropomorphic Personification of Death.

Blast is...
  • ... Dead All Along. He's so reclusive that it wouldn't appear out of the ordinary if he didn't show up for a while.
  • ... not real. He was made up by the association so the higher ranked S class heroes would have a goal to work hard for. Any characters claiming they've encountered the guy have actually met an actor playing the role. Probably Jossed, as Tatsumaki met him 15 years before the Organization was founded.
  • ... just as strong as Saitama. Because Victory Is Boring, he's resigned himself to accept that nothing will ever challenge him and given up on heroics altogether, explaining why he never shows up.
  • ... a woman. Specifically a Distaff Counterpart of Saitama, and also bald. Really doesn't seem so. He looks quite masculine in Tatsumaki's flashback.
  • ... a Person of Mass Destruction so powerful he could potentially destroy any threat... Earth itself included.
    • Building off that, Blast is a pyrokinetic on the level of an (adult) Charlie McGee.
  • ... Saitama. He was given the #1 S-class position and title of Blast by the butt-chinned man who started the Hero Association, but couldn't find a way to contact Saitama (due to not knowing his name) to inform him. Thus why no one ever sees or knows anything about the man named "Blast". Most likely Jossed.
    • Someone who looks like Saitama did back then, got mistaken and just rolls with it. That's why he never shows up for heroic deeds. the flashback says Tatsumaki met Blast 18 years ago. However, Blast could be Saitama from the future what with the similar costumes, albeit with a few differences. It's kinda hard to tell since it hasn't been redrawn yet.
    • Plus, his Kirk Summation to Fubuki sounds strikingly similar to BLAST's advice to Tatsumaki years ago, about not relying on weaker people and how eventually they'll meet someone stronger. I mean, look at this, parts of it are practically word-for-word the same:
      Saitama: (to Fubuki) ...When a monster stronger than you shows up, none of your subordinates will save you...
      Blast: (to Tatsumaki) When the time comes, don't go expecting someone to come save you.
  • ...Bang and Bomb's third brother, following the same naming conventions. Unlikely, given how much younger than them he seems in Tatsumaki's flashback but still possible
  • ... a cyborg. Specifically, the cyborg who destroyed Genos' hometown and killed his family.
  • ... Metal Bat's younger sister, Zenko. Jossed.
  • ... someone Saitama might never be able to defeat should he appear one day.
  • ... never around because his training made him bald and he's too ashamed to be seen anymore.
  • Saitama's father. The reason he's never seen is because when Saitama's gained his powers, Blast lost his.
  • This guy. He does bear some resemblance to Saitama before he became bald.
  • ... actually a spirit that possesses a host, lending his host tremendous power. He was in an old host when he helped Tatsumaki but degradation of his host's physical form forced him to move on to a new body. The identity of that new host? Saitama. Blast needed to find a new host right when Saitama was training to become a hero. Blast possessing Saitama is the real reason why Saitama became ill during his training, why Saitama's hair fell out, and why Saitama became so strong. Also, the ultimate final villain of the series will have been trying to exterminate Blast, who has been his enemy for who knows how long.
  • ... Mumen Rider. Mumen never shows his face, but has been seen with hair similar to Blast. Perhaps Blast is Saitama's next "phase" or an inversion - so ridiculously powerful that he wishes to be normal again, resulting in a total loss of powers. He still needs to be a hero, though, so he hangs out in Class C. Probably jossed as Mumen Rider attended the same school as Saitama as a kid. At that time, Blast would have been an adult.
  • ... a fraud. He's not actually that powerful and most of his "heroic" deeds are either rigged or the result of him stealing credit for the work of other heroes. He stays out of the spotlight to avoid arousing people's suspicions.

One-Punch Man and My Hero Academia are set in the same universe, just at different points in time—and Saitama is a holder of One For All
  • Think about it: Very little explanation is given as to why some people have certain powers and why others don't in the One-Punch Man universe, but they all seem suspiciously consistent with MHA's explanation of their power (one of the Hero Association's heroes can bend metal at will, and sister heroes both have telekinetic abilities); sounds an awful lot like the "quirks" of MHA to me. It should be noted that MHA tells us when the first quirk was recorded as BEING a quirk and the acknowledgement that such things officially existed, but how long they existed before that is never explained. Saitama and All-Might have notably similar powers (and similar lack of powers) and demonstrated power levels on par with one-another. Their punches can cause extreme weather distortions, neither can fly, but they can jump exceptionally well, and are both virtually indestructible (except supposedly against blades, which we haven't seen how Saitama reacts to being hit with a blade). Also quite similar are the criteria for being able to inherit One For All: a body in extroardinarily good shape. Saitama credits his sudden power gain to three solid years of strength training (100 push-ups, sit-ups, squats, and 10km runs every day), whereas Izuku Midoriya (the protagonist of My Hero Academia) conditioned himself by cleaning a beach filled with all manner of heavy items and things in addition to jogging, weightlifting, and swimming over the course of 10 months. Izuku has trouble controlling One For All at first because he "was a vessel made in a hurry", whereas the less intense workout over a longer period of time by Saitama allowed him far better control. One final note: in the One-Punch Man canon, the Hero Association has only existed for about 3 years and its competance is questionable at best, whereas the Hero Offices of MHA are generations old and have heroics almost down to a science. Is it possible that the Hero Association eventually matured into Hero Offices?
    • Impossible. One-Punch Man has been confirmed to take place in a world with only one continent, separated into 26 districts. My Hero Academia takes place in a world with geography similar to that of the real world.

Whatever power Saitama has, it's NOT in the OPM universe
  • If the word is going to be believed, a full-force punch of Saitama would contain half the energy of the Big Bang. Theoretically speaking, that would mean Saitama would contain more power than the force used to create the universe. Since a system cannot contain more power than it's needed to create it, his entire universe would have been crashed by the mere presence of him (a system would have to account for potential energy that's always existing). Since that isn't happening, as we can see, that mean the source of his power must come from outside his own verse, probably via a pathway that he could control the output. (Which would explain the delicate control he has to live an ordinary life)

(Again, that's assuming this is and it's half the energy used to create his universe (if it's bigger than our))

Handsomely Sweet Mask is an Ugmon
He's the same type of being that becomes a monster due to their appearance except unlike normal Ugmon's who become uglier, he became handsome but his heart got uglier.
  • Confirmed. He thought he was ugly so he eventually became beautiful, sort of like a reverse-Ugmon.

The grandfather of the kid from the 2nd chapter
Is a lovechild of the Ballchinian.

Multiverses exist and the real source of Saitama's strength is similar to The One
The risks that Saitama took (fighting more powerful monsters, training from hell, poor diet, etc.) in his time as a hero killed all/most of his multiverse selves, leaving his remaining selves/self all-mighty. This is why he has become super-powered while doing mundane training. Alternatively, all of the other Saitamas have died while training or fighting monsters and the one we see is the lucky one who has been absorbing the power of the others without even knowing it.

The story will end with Saitama either lose his power (and regrow his hair) or his death.

Genos is the android who destroyed his village.
Years and years ago, Genos was a villainous android who decided to attack a village of defenseless people. Dr. Kuseno, being a genius, manages to incapacitate him and decides to rehabilitate him, realizing how useful that much power could be to the world. Genos has false memories implanted within in his head (that he grew up in the village he attacked) and is awoken by the doctor to fulfill his new role as shackled protector. However, Kuseno is surprised by the androids apparent age and reverent attitude and grows to care about him.

The story of the other android came later when Genos asks where his family is, it was something Kuseno makes up to avoid the truth.

  • As noted below under the "Genos was never human" guess, jossed since if that was the case, we wouldn't have seen any muscle on his face/spine or ribcage when the Deep Sea King's acid melted him, meaning he's a cyborg that's more robot than human at most.

Sweet Mask is going to kill a main character at some point.
He's already a complete psychopath, and his Sanity Slippage post-garou is making an already bad condition worse. He's also clearly not entirely human, as judging by his regenerative abilities, so it's possible his crossing of the Moral Event Horizon may be inevitable. What better way to both destroy any remaining audience sympathy and ensure Saitama challenges him seriously than by killing one of his allies? He could even pull an Adrian Veidt to be extra nasty.

The whole series is Saitama's dream
Near the beginning of his training, he overexerted himself and passed out, and everything since then has been a dream. In the end of the series, he'll easily defeat the ultimate enemy, then wake up, and realize that maybe being so strong isn't such a good thing after all, so he gives up on his training and goes back to being a normal person.

Genos was never human
There is no destroyed hometown and massacred family, there is no robot bent on destruction: it's all a lie. Dr Kuseno who is a secretly a Magnificent Bastard bullshitted any of Genos' memories of being human in order to give him empathy and other human-like emotions in order to prevent him from going rogue.
  • Jossed. After Genos is melted by Deep Sea King's acid, you can see some muscle on his shoulders and face as well as a spine and a ribcage. He also seems to be crying soon after being defeated.

Saitama's power is unconscious gravity/mass manipulation
Lots of little things me to this idea:
  • His training regimen never got easier, he unconsciously increased his mass/weight so it would always push his limits.
  • Violations of laws of motion, when he causes mass destruction in one direction there should be an equal and opposite reaction, there isn't
  • Tatsumaki remarked on how heavy he is

Boros's prophesied opponent wasn't Saitama.
Boros was promised an enemy who could fight him on equal terms. The problem is, him and Saitama aren't equal. Saitama is thoroughly out of his league, and the only reason Boros lasted so long in their fight was because Saitama wasn't bothering to seriously fight back.

Boros's promised opponent wasn't Saitama at all. He was supposed to fight Garou.

  • Of special note, Word of God has stated that a fight between Boros and Garou would be one hell of a battle.

Tatsumaki would be a Shipper on Deck for Saitama and Fubuki
  • Because a) She's really overprotective on her sister and b) her mind jumped into the strangest conclusion.
  • Saitama already declared himself as neither underlings or friends of Fubuki. There's no way Saitama would let her do what she wanted, and she had no chance against Saitama. She would lose against Saitama, but then wondered why he'd go so far just for an 'acquaintance'. She would take a wrong conclusion that Saitama is Fubuki's 'special person'. She would approved it since Saitama's strong enough. Hilarity Ensues.

Sonic will become Saitama's equal...
  • Sonic's desire to beat Saitama will cause his body to transform like the mysterious beings who become monsters due to something like eating too much crab. He'll gain Saitama-level strength and speed but will become bald too.

Saitama will kill Handsomely Masked Sweet Mask.
  • Saitama and Sweet Mask will fight at some point but Saitama will see that Sweet Mask is more monster than human and will kill him with one punch like usual.

Metal Knight is/invented the android who destroyed Genos' village.
He seems to care more about making strong weapons than protecting people, so he's probably the kind who wouldn't care if one of his experiments went out of control and destroyed a village. The way Drive Knight specifically warned Genos about him makes me think they're being set up as opponents later on. Plus, before meeting Saitama Genos was a lot like him, only worrying about becoming stronger and better, so they could be foils.

Saitama is a Kryptonian.
He was always abnormally strong, as seen when he dealt with the crab monster, Crablante. However he was constantly wearing long-sleeved shirt and pants, so most of the sunlight was wasted on his clothes. Therefore he only had low-grade kryptonian abilities. Once he went bald, his head absorbed a large amount of sunlight, which was previously stopped by his thick hair. From that point on he started developing kryptonian super strength, speed, durability and super senses (as seen in the webcomic).He can also use other kryptonian abilities such as flight subconsciously, as seen when he kicked back a huge several tons shell back at boros's ship while being airborne, or when he jumped back from the moon to the earth and landed directly on the ship. However he is absolutely sure he is human and as "Humans cant fly" he never actualy tried flying. Same goes for heat vision and ice breath, he simply never tried them, as he is convinced he is human.

Saitama will eventually fight against a villain with Intangibility.
The Logical Weakness of someone who can end fights with one punch is being unable to land a punch on his enemy. Thus, Saitama would probably have trouble with an intangible man...until he punched the intangible man's feet, which are conspicuously not falling through the Earth. Or maybe someone else pulled a Big Damn Heroes on Saitama for once? There's a lot of interesting directions ONE could take this in.
  • Maybe he won't consider it a fight, unless the intangible person has other powers to help him fight then their won't be a point for them to fight. Saitama wants to fight someone who needs multiple hits, not a someone who he can't fight.
    • Or rather, he wants to fight someone who can actually hurt him. In his dream sequence, he was killing the Subterraneans with single punches, but they also managed to hurt him as well. Saitama wants to feel pain again.

Saitama will die after using his full strength.

Tatsumaki is the foil of Saitama.
They have some similarities.
  • Both have the ability to change art styles
  • Both look non-threatening, but have incredible power.
  • Both have a weaker student(Fubiki and Genos)
They also have tons of differences.
  • Saitama had a normal life before being a hero, Tatsumaki had a traumatic one.
  • Saitama is calm, Tatsumaki is constantly moody and easily annoyed.
  • Saitama is low ranking and gets no respect, Tatsumaki is ranked #2 in the association.
  • Saitama fights entirely physically, Tatsumaki uses her psychic powers.

Genos doesn't give out his specs specifically so nobody will analyze them.
When registering with the Hero Association, Saitama offhandedly asks if there's even a point to Genos participating in the physical exams, if instead he could just give the Hero Association his technical specifications. Genos is hesitant to answer and decides not to, while Saitama waves it off. Later, we see Metal Knight test-firing his new weapons, then Genos being told Metal Knight is not an ally, and finally Metal Knight analyzing Boros' ship, using the data to reinforce the Hero Association's headquarters. It may well be that Genos consciously decided not to give out his specifications because he worries that it can be replicated or analyzed for weaknesses by his enemies, and is unknowingly protecting himself against Metal Knight, who might be the cause of his trouble past.
  • Alternatively, Genos doesn't give out his technical specs because he doesn't know what they are. He knows the limits of his abilities and strengths in combat, but as he isn't a mechanic or roboticist or anything like that, he just trusts Dr. Kuseno to repair and rebuild him.

The God who gave Homeless Emperor his powers is the one who created Vaccine Man
Homeless Emperor and Vaccine Man both wanted to Kill All Humans because they saw them as threats to the Earth, and they both have similar ball of light attacks.

Blast and Saitama are related
Given how Tatsumaki's flashback showed that Blast has a lot of Saitama's traits, mannerisms and even shares a resemblance in appearance it's entirely possible Blast is either Saitama's father, an uncle, or older brother. If not any of those they have to be related some way.

License-less Rider scored exactly the opposite of Saitama on the Hero Association exam
While Saitama scored perfectly on the physical examination, his utterly lackluster motivation (to be a hero for fun) caused him to score poorly on the written exam, which knocked him down to Class C. License-less Rider is Saitama's Foil: physically unremarkable, but his perfectly heroic personality and intent allowed him to ace the written exam and attain a Class C rank.

Licenceless Rider has the potential to One-Punch
If we accept the theory that Saitama's power comes from his constant workouts, due to his obsession with being the strongest, then it's logical to assume the constantly on-the-go Licenseless Rider has done all the training required. Furthermore, he and Saitama are the only ones who regularly express a belief in doing the right thing for the sake of helping others (with LR lacking Saitama's more selfish attributes). Therefore, its only because he doesn't have that obsession with being the best that he can't do it. If he did, he'd be able to win any fight with a single punch.
  • Not a punch. With all he cycles, he would probably be better suited for kicks.

Saitama's power is a result of him transcending the level of anime hair and going bald.
As we all know from shows like Dragonball Z, the longer, wilder, and more fearsome a person's hair is, the greater their strength. In training, Saitama achieved Saiyan level of power, and transcended it; but the strain was too much on his hair and it fell out. The level of power in his body is too much for his hair to register. Now he is permanently stuck at a level of power that surpasses even the Saiyans, doomed to never have a hair on his head again.

The secret behind BLAST's rank and power
Like Saitama, he trained himself so hard he developed superpowers that outclass every other S-Class hero combined. His workout regimen?

99 push-ups, 99 sit-ups, 99 squats, and a 9km run, every single day. Except Sundays.

If he'd just pushed himself a little bit harder... well, who knows what would've happened?

Saitama's power is the ability to one-hit kill anyone or anything.
The ability manifests itself as super-strength, -speed, etc. because it's based on Saitama's impressions of superpowers from the anime/s he saw as a kid. If it had been a different person (say, a videogame nerd like King), that person would have gaming-themed superpowers, for example (simmilar possibly to Han Jee-Han's?). Thus, it's a sort of eldritch power, and maybe You Cannot Grasp the True Form.

A side theory could be that he's had this power dormant inside him since birth, only surfacing when Crablante attacked, which drove him to become a hero and find a "proper" (so to say) way to channel it (=letting it manifest as typical Flying Brick-type superpowers).

(Up for debate if Saitama genuinely wanted to be a hero independently of his powers or if they subconsciously influenced his decision, but if one sided with the latter, then one could postulate Saitama's problems with heroism [such as lack of popularity, badmouthing by literally everyone, etc] as fate/destiny/the Powers That Be/what have you deliberately screwing with him so as to stop him from being a hero, since he is by far too OP.)

Saitama is a masochist.
Despite Boros being the strongest fight Saitama had to deal with so far, at no point does he show he's enjoying it. While in the dream fight with the subterranean people and the fight with Crablante, he is show to have a fire in his eyes. In the later two fights, he defeats his opponents in one hit. Why does he enjoy these fight more. Because both opponents hurt him. Siatama isn't actually searching for an opponent who can survive multiple hits, he wants an opponent that can hurt him.

Saitama has brain cancer.
Saitama has brain cancer, and is undergoing chemotherapy. That's why he went bald. All of the his of the adventures are hallucinations.

Saitama is the reincarnation of Goku or Gohan.
He does similar training, and receives a similar power up. This is Saitama awakening the power he had from his past life.

Saitama is dead.
Not in the sense that he's in an afterlife, but rather that his training killed him and what we're following is his corpse animated by his own willpower.

Psykos is feigning loyalty to the Orochi
If the Monster Raid Arc is a prelude to the Garou Arc, Psykos would have to be the leader of the monster association by the time it happens. Maybe she's already foreseen that Orochi will be killed in the near future (probably by Saitama) and hasn't told him because she sees his death as a chance to seize command for herself.

People from other fandoms who could actually give Saitama a legitimate, straight fight

The series will end with a Title Drop
Specifically, it will end with Saitama getting the recognition he's been looking for within the Hero Association. He'll be promoted to S class, and his hero name will be changed from "Caped Baldy" to "One Punch Man".
  • I think the intro is the description of Saitama after he was revealed to the whole world. He looks serious during every fight, there is no goofiness involved. And the constantly changing setting may imply, that he decided to walk the Earth to find a purpose.
  • The title in Japanese transliterates directly to "Wanpanman," which is clearly a pun on Anpanman. It doesn't discount this guess, but it suggests there is a reason for Saitama not being known as such in-universe other than it'll come up later.

Puri-Puri Prisoner is a Yaoi Fanboy
Would THAT be so surprising?

One reading of how Saitama got his godlike abilities, given the very Buddhist connotations of his baldness and training regimen, is that his unwavering commitment to being a hero solely for the sake of being a hero is where his powers came from.

Saitama's powers were granted by God
As in the creature called God by the Homeless Emperor. Combining its ability to grant vast amounts of power at will as well as its ability to do this and communicate through psychic links, it could well be God is the source of most of the monsters in OPM's world. This WMG assumes God's psychic powers make it aware of humans with spectacular levels of obsession with something. Be it crab for Crablante or being able to destroy evil in One Punch for Saitama. Banking on the idea that obsession breeds greed or some similar Humans Are Bastards philosophy, God grants powers and transformations to these humans in-line with and in proportion to their obsessions. So a gluttonous man obsessed with eating crab becomes a crab-monster and goes on a rampage. God hoped Saitama would have been the ultimate monster, becoming drunk on power and scornful of his new appearance to the point of causing unstoppable mayhem. Instead, Saitama's transformation was turning bald and he just became bored with his strength instead of using it with abandon, as he just wanted to fight monsters instead of be strong for its own sake.

The final major battle of the series will be Saitama vs God.
Again, the being Homeless Emperor calls God. A being that is able to grant and tear away vast amounts of power with ease, as if powers like creating tiny stars that explode on contact are just a drop in the ocean of its strength. It's very easy to assume it will be OPM's first nad possibly only God-level threat, because it is named God. This will seem like the fight Saitama has dreamed of, where the real threat of injury and death will make him feel alive again. Unlike most if not all fights in OPM, this fight won't just be two characters throwing themselves at each other in kickass ways. It will be a more traditional Shonen fight, with Saitama having to actually think about how he uses his strength and come up with tactics and mindgames to overcome God's strength. As well as the emotional throughline not being about Saitama being dissatisfied that he's found another Boros, but that for the first time in his life he is scared witless and doubting his ability to win a fight, because he is fighting God, who I assume is the very being who gave him his powers in the first place.

The opening shows how people will remember Saitama after the show
I always wondered why is his expression so grim while fighting enemys he can oneshot. I think after he finally receives the praise he deserves, he will find it boring, and starts wandering hte earth searching for a worthy opponent. The opening shows how the people remember him, a grim hero who does not enjoy fighting.

Mumen Rider and Saitama will get a role reversal at some point
at some point there will be a monster that simply cant be beaten by physical force but rather some metaphysical thing can hurt it, perhaps it is some immaterial demon, or maybe some being protected by a prophecy that only the greatest hero could defeat it, so it incapacitates all the S class heros thinking its safe, either way the remaining heros all attack it only for there attacks to predictably do nothing,, eventually saitama shows up and punches it, only for it to have no reaction at all, of course while saitama cant hurt it hes still ridiculously durable, so its down to him to keep it destracted and jump infront of its attacks to keep it from hurting the more fragile heros, till Mumen Ride shows up, and threw being pure of heart, being the greatest her by definition of most heroic rather then most powerful or some other trait of his manages to destroy the creature in one punch, he might even return the favor of giving saitama(and the other heros) credit for takeing it down so he finally gets the credit he deserves.

One-Punch Man is set in the same universe as Empowered
Many, many superheroes? Check. Many heroes being jerks? Check. A form of organization for the heroes? Check. It's the same setting, with One-Punch Man being in Japan and Empowered in the US.

Saitama is so strong because he accidentally summoned the spirit of Bud Spencer
Specifically, he summoned all of his characters. And considering what just one can do...

Saitama is a glitch in the universe and OPM was supposed to be a Shonen featuring Garou and Genos with Boros as the Big Bad
Adding to a WMG above, Saitamas training should have killed him but instead the universe glitched and he lived becoming infinitely powerful. If things had gone as they should have all the monsters he would have killed were instead brought down at a much higher cost by weaker heroes. By the time the OPM story starts the world would probably be in some pretty dire straits.

Enter Garou who in a timeline without Saitama is in a world where people and heroes are constantly being killed by nigh unstoppable monsters. Instead of being a hero hunter he becomes a monster who hunts monsters and seeks to be the strongest. He is joined by the Demon Cyborg Genos who seeks revenge and justice.

Over the course of the story Genos eventually begins to sway Garou into actually just being a hero by the time the Boros arc hits. Garou fights Boros into his second form before awakening to the power of justice. Finally giving Boros the fight he was looking for.

Saitama is actually an NPC
We all know he supposedly gained his massive power through his "training" which can't even be comparable to workout regimens for professional athletes or other heroes in the world itself who don't have nearly his strength. But think about it, he doesn't have any of the "special" aspects that most of the other heroes and even villains of his world have. He has no grand goal in life. He has no tragic backstory. He has no real desire to help people. He was a regular salaryman who just decided to become a hero as a hobby for the thrill of the battles he'd get to fight. He only decided to become a hero after he happened to save someone during a monster attack once, meaning that he was one of the normal citizens needing to be rescued at one point. And that's just it: he was a regular unimportant nobody. He was an NPC. It's been mentioned that he broke his limiter and gained such a level of power because of his training, and while the training may be lackluster for the real world or even other heroes in his world, for an NPC, it was above and beyond what any of them should be capable of. Non Player Characters are generally there to either fill the background or be killed off to show how much of a threat something is, either in terms of power or just how evil they are willing to be by killing innocent people. Saitama was an NPC who stepped out of his role as such. He went beyond his limits as a background character. That's why he's so powerful.

One-Punch Man takes place in the same universe as Heroic Age and Saitama is either the reincarnation or the direct descendant of Age and Dhianeila .
Saitama and Bellcross are almost similar since they're pretty much an unstoppable being that could survive anything that is thrown at them and ends almost any of their fights by just one punch. Saitama's source of power is probably comes from the Nodos of existence itself and he is unaware that he is actually bend the reality just like Bellcross

Orochi is only a meat puppet controlled by Psykos to help her lead the monsters.

One day King will be forced into a battle he can't bluff or wriggle his way out of.
It will be against a Mysterious Being who can only be defeated through a video game.

Saitama does have powers, but they aren't combat related
Instead, they drastically magnify the ability training has on him, meaning that a normal person could be as strong as him if they did his workout equal to however many times his power multiplied his memetic training regiment.

The B-Class would've eventually become stronger than the A-Class and not even realized it.
Important disclaimer: NOT. COUNTING. SAITAMA.

They were led by someone who would easily make it into the top 10 of A-Class, and it's made a noticeable point throughout this Wiki that her keeping the B-Class #1 spot holds back others from advancing to A-Class as well. It's entirely possible that a good chunk of the top ranks of the B-Class was stronger than the bottom of the A-Class before Fubuki made her decision to finally advance.

Saitama is a Normal Fish in a Small Pond
The city is just pathetically weak. The reason for the limiters is to allow weak people to live in peace.

The organization that held Tatsumaki prisoner was Claw
Would explain everything about her, especially the psychic part.

Kuseno was the one who created the cyborg that destroyed Geno's village.
I find it way too convenient that Kuseno a master of cybernetics just showed up. I think he created the rampaging cyborg and feels at least somewhat responsible for whatever went wrong during the body modification and is now hunting the cyborg down

King's (other) Power is Fear
King is a Project Empath; he can channel his fear into others.

Saitama can fly, he just doesn't know it
Some of his moves, such as changing direction quickly in mid air, would only be possible with aflight like ability. But because he doesn't think he can fly he doesn't do knowingly.
Saitama’s power comes from the Getter Rays.
During his three years of training, his Hot-Blooded desire to become the strongest hero caused the Getter Rays to become attracted to him endowing him with their infinite power and causing him to hyper evolve to a state where humans would be universally bald. The reason that when he punches with the intent to kill the victim sometimes explodes into Ludicrous Gibs is because in addition to augmenting his physical strength, the Getter Rays are occasionally release themselves into his opponents body, triggering mutations in several cells, causing them to become cancerous and causing the enemy to explode. Carnage Kabuto became terrified of Saitama as soon as he sensed something off about him, much like the villains in Getter Robo have an innate fear of the Getter Rays. Not to mention during the theme song the lyrics are “Power! Getter power!”

There'll be a character who is based on Chirin.
Why? Since Saitama is based on Anpanman, this character will be on Chirin who has a similar (somewhat) personality and power to Saitama, but is a antagonist and villain who's only being a villain for fun and as a hobby. It be only natural since they are both based on the characters that are created by the same person, Takashi Yanase.

Saitama will come up against a monster who can take his consecutive serious punches.
At which point he'll unleash his super serious punch.

Saitama is the threat to humanity in Shibabawa's prophecy
Saitama will eventually face someone powerful enough to give him a fight. However, if he goes all out against them, their fight will destroy the world. Saitama will be faced with the dilemma of finally finding fulfillment and keeping the world safe - he won't be able to have both.

Metal Knight and Kuseno are probably related.

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