Ma'am Shock: Was more pronounced when "ma'am" meant an older woman. Now not as big of an issue - except for men mistaken for women on some occasions.
MacGuffin: The Bragging Rights Reward in its physical permutations (e.g. trophies and objets d'art) and anything useless that only has value in being collectible and/or desired by more than one person.
MacGyvering: VERY common in Real Life, especially among people who are cash-poor. Everything from furniture creation to computer repair to fashion to automobile repair to animal and human health care may well be subject to it depending on the setting.
Machiavelli Was Wrong: Once a certain level of strict, iron-fisted rule is passed, said authoritarian rule is often (though not always) more likely to result in widespread disobedience of the rule at least in secret, to open, violent rebellion against it. The most successful way to maintain a complicit populace seems to be offering a mixture of reward and punishment, with just enough reward to keep people seeking more and just enough punishment to make rebellion or schism unappealing. North Korea seems to be the only exception to this rule.
Machine Empathy: People get used to the noises and rhythms of things they use often. Whether it's a vehicle or a computer or a musical instrument or whatever, the first sign something is wrong with it is often something noticed only by the primary user...
Made of Evil: Some individuals throughout history who seem to have taken enjoyment only out of how much they could directly and indirectly harm people and/or the environment, and societies constructed on the concept of racist hatred and genocide and killing. No more will be said, and no real life examples (except forAdolf Hitler and Nazi Germany) should be discussed because what people and societies are such is incredibly subjective for the most part.
Made of Explodium: Hydrogen dirigibles, nuclear fission or anything capable of it, a lot of fertilizers including the infamous ammonium nitrate, rocket fuels and other high-energy fuels, boilers and steam equipment that isn't properly managed... and many, many more things that can create a huge blast without much trying.
Made of Iron: Some human beings, either through intense training and physical strength/skill, tolerances, or just plain dumb luck or some combination thereof, survive things that should kill them. Drunk people sometimes tend to survive situations sober people don't, especially in regard to accidents and falls. Babies are another example - while Infant Immortality is sadly untrue in real life, babies are fairly tough and can survive situations adults generally can't.
Major Injury Underreaction: Shock can cause this as well as intense anger or fear, and/or being drunk or high. Not understanding the severity of an injury is another reason. Injuries involving the brain are VERY bad for this since stroke or traumatic brain injury sufferers are often oblivious to the extent of their injuries. Some internal injuries such as abdominal trauma or a collapsing lung aren't spotted until hours after they were sustained. Finally, people who are used to a common level of pain from existing injuries might not get the severity of a new injury - for example, someone with neck injuries that cause intense pain might not, especially in combination with other causes, even notice a broken neck as anything but an especially bad pain flare and sudden inability to keep pushing through the pain.
Make the Dog Testify: Lawyers actually can and do file motions to declare animals, plants, or even objects witnesses or demand they be brought before the court - it isn't usually intended to put the dog (or the plants, or the object) on the stand - the intent is usually to acquire evidence with the permission of the court (e.g. to DNA test the dog when the owner in a civil suit insists it wasn't his dog that got the neighbor's dog pregnant, or to sneakily read communications stored on the object, or to see if the person's alibi that they were watering the plants when the crime happened was likely true or likely false)
There's also the very controversial practice of civil forfeiture, where a suspected (read: not even convicted) offender's property is itself listed as the defendant and taken. If you've ever seen a case like The People Of The State Of Florida Vs. Ten Gold Chains, One Lamborghini Aventador, and Five Bottles of Belvedere Vodka, this is likely such a case.
Maligned Mixed Marriage: There are hundreds, if not thousands, of examples throughout history of people hating those who marry outside of their race. In fact, at one time in the US, there were laws banning miscegenation note interracial marriage.
Mama Bear: Both the animal and human kind. There are documented cases of mothers rescuing their children from mountain lions and actual bears.
Mama Didn't Raise No Criminal: The families of real life criminals do indeed live in this sort of denial sometimes, even when confronted with overwhelming evidence of criminal behavior.
Manipulative Bastard: Sociopaths and malignant narcissists are experts at being this, and some people with borderline personality disorder accidentally become it or become it to protect themselves from suffering. And then there's people who may not have any specific mental disorder but just like to control other human beings.
Manipulative Editing: A staple of Reality TV, and also a very crude and deceptive way to put words in someone's mouth or make them support or not support something against their wishes.
Manly Tears: There is nothing weak or un-masculine about having emotions other than anger/hatred/rage and lust.
Marijuana Is LSD: Generally NOTTruth in Television, but sometimes dealers will "wet" cannabis with PCP or another smokable hallucinogen, and/or pass off "fake pot" such as K2 or the JWH compounds as real cannabis. Also, eating sufficiently large amounts of cannabis can have a far different effect than smoking - the "body high" effect of eating large doses can sometimes be likened in similarity to MDMA or LSA.
Marital Rape License: Unfortunately in many parts of the world for a very long time. Thankfully, in much of the world after the late 20th century, revoked - and in more places by the day.
Marriage to a God: Nuns and priests and various celibates within Christianity, though they would say to the God. Other religions provide many more examples.
Marry for Love: In most Western countries after the mid to late 20th century, the reason for many marriages - as in, the Arranged Marriage and the marriage for financial reasons only began to recede in popularity. This may change with the economic crash of Turn of the Millennium and The New Tens, where people are expressing more willingness to marry (or to get into relationships that aren't marriage) for places to live or for money.
Masculine Girl, Feminine Boy: Such relationships have existed and do exist, because not everyone, even in the most highly gendered societies, likes to stick to the rules.
Maternally Challenged: Because being physically capable of concieving and giving birth to a child does not mean one can be or will be a good parent. The saddest examples will end up on the news every so often.
Maternity Crisis: Occasionally, but fright/terror doesn't induce labor as often as it does in fiction.
Matter of Life and Death: Quite a few things in real life are exactly this - lives or at the very least lack of severe injury or property damage are at stake. These things are where Serious Business is absolutely, thoroughly justified.
Parents naming a child for someone or in memory of someone or otherwise a name they've chosen for meaning.
Some transgender people, when selecting their new name, will pick one that is meaningful rather than just a masculine/feminine variant of their current name or a Gender-Blender Name.
Changing one's family name/last name to one's partner's family name/last name on getting married.
Some people converting to Christianity or Islam will choose a "Christian" or "Islamic" name.
Some witches and other occult practitioners choose a "magickal name" or "secret name."
During the Civil Rights Movement, Malcolm X chose the "X" in his name to represent the now unknown name stolen from his family by the slave masters who ultimately forced the name "Little" on his ancestors. Renaming for these cultural reasons, e.g. taking on African names, was not uncommon as a result of the same feelings.
Meat Versus Veggies: There are many cases where persons debate with one another about the superiority of one kind of diet over another. If you're a vegetarian, you have probably had to contend with all sorts of reactions ranging from concern borne of ignorance to open scorn or derision. Alternately, if you're a meat eater, you've likely been accused of everything from murder to animal cruelty to endangering your health.
Men Don't Cry: While male brains do seem to have less activity in areas related to emotional understanding and similar, and testosterone does have some antidepressant/emotion regulation effects (specifically toward enabling the "bottling up" of emotion over sudden uncontrollable crying for example), it is still not that different, especially in the absence of mental illness or in the presence of actual tragedy, and this trope is only true when and where it is because of gendered societal conditioning of men playing up this "attribute" and demanding it above and beyond all else.
The Men First: Insofar as their instructors can hammer it into their heads.
Middle Child Syndrome: Sometimes, especially if the middle child is average/normal enough and/or very easy to raise compared to, say, extremely rebellious or special needs children, or on the other hand, an extremely gifted child that needs constant intellectual stimulation to be provided.
Minor Injury Overreaction: Overdramatic people or people looking for a Frivolous Lawsuit often do this. It's also a result of injuries that while technically minor, look or feel worse than they are with lots of bleeding for a few minutes or by being excruciatingly painful due to oversensitivity to pain, the injury happening in an area with very high pain sensitivity such as the genitals, the hands/wrists, or the face/mouth/nose.
Misleading Package Size: Overpackaging, fraud, or the desire to present something as more valuable than it is can make this happen.
Misplaced Wildlife: A serious problem in many areas of the world, especially if plant and insect species are counted as well. It's called "invasive species," and if you've ever been stung by a fire ant or Africanized bee or had to clear kudzu, you've seen them.
Another variant is when humanity invades on a former wildlife habitat, either temporarily with camping or hunting for example, or permanently with housing developments and the like. Deer, bears, boars, or whatever else was in the area before humans began to encroach will still be there... and attracted to things like food in garbage and movement of people walking.
Miss Conception: If sex education doesn't exist. More common in the past, still sometimes happens in restrictive religious communities.
Missed The Bus: Annoying for people who are dependent on public transportation - missing a bus or a train often means waiting anywhere from 30 minutes to two or more hours to get to one's destination.
Mission from God: From Joan of Arc to Osama bin Laden, religion has been a powerful motivator.
Mistaken for Cheating: With some of the more jealous and paranoid people one can be in a relationship with. If this happens on a regular basis, it can be a symptom of abuse, distrust that is absolutely irreconcilable, or simply of a relationship you'd be better off without.
Mistaken for Junkie: Anyone can be and there are quite a few groups who regularly are: diabetics (needles, occasional odd behavior), transgender people (hormones being technically controlled substances, needles, hormones needed to maintain gender), the mentally ill (occasionally odd behavior, dependence on prescribed medication to be normal), people with chronic pain (use of drugs junkies do use, but legally prescribed and medically maintained for a legitimate reason), and nocturnals/persons with sleep issues (often mistaken for users of illegal stimulants).
Mistaken for Terrorist: Unfortunately happens, and not only to Middle Easterners, but to photographers, hobby chemists, people who as much as say the wrong words in the wrong sequence or the wrong place, and to many others in the US.
Mix-and-Match Critters: Ligers (lion/tiger), mules (horse/donkey), goat/sheep, cattle/bison, zebra/horse, and pretty much any match within feline or canine species (e.g. mountain lion or panther/domestic cat, coyote or wolf or dingo/domesticated dog) are a few of the hybrid animals possible in real life. Many are considered undesirable by humans, but mules are valued for strength and docility and sterility - making them perfect pack animals - and wolf hybrids for intelligence and endurance and size - which with the right owner makes them perfect working dogs or guard dogs.
Mock Millionaire: Quite a few fraud artists have done this. Also advice from the "fake it until you make it" school of celebrity or financial success, which preaches this as a lifestyle - to consume intensely, to "live what you want to be."
Modesty Bedsheet: You've probably used this if you sleep nude and someone unexpected enters the room.
Modesty Towel: Force of habit and shyness will often lead to this being used even when there is no reason for it. Paranoia can also be a factor, if your room has a window.
Monster Mash: IfOtherkin and/or some who believe in the paranormal actually are on to something. In a more concrete, realistic example, Halloween parties and cosplay events where there are "monsters" and paranormal creatures of all sorts represented.
Mooning: College campuses and anywhere else people are drunk and stupid enough to think showing off their ass is a funny joke. Not a good idea - doing this in real life can get you slapped with a public indecency charge and a spot on the sex offender registry, even if it wasn't intended as sexual harassment.
Morning Sickness: Happens to some pregnant women though not to all, and some have it far more severely than others.
Motive Decay: Remember when you wanted to be an astronaut or a firefighter or a star? Look around at what you are now, and if you gave up on that goal for whatever reason (good or bad), this has happened to you.
Also tends to happen to political and social movements - where originally the cause or principles may have been important, but eventually the political or social movement lost sight of its exact mission and ended up with 100 different goals it cannot accomplish at once or worse, ended up committed to maintaining its hold on power at any cost and with no other real goal.
And also common among criminals - which is part of why many become Stupid Crooks.
Mrs Degree: A sexist stereotype that, unfortunately, does have some truth - some women do go to college mostly to seek not their own education, but to land a doctor or lawyer as a husband.
Mr. Vice Guy: Pretty much everyone, as there are few people (if any) in the world truly devoid of any vice.
Mugging the Monster: If people make a career of robbing people, there's always a chance they'll pick the wrong person to try to victimize and end up getting the shit beat out of them or shot. There have been quite a few cases of muggers trying to assault karate students.
Mundane Luxury: To someone living in a third-world country, simple things like clean running water can be considered luxuries.
Multi-Track Drifting: Because some vehicles can be pushed beyond their usual limits, with an experienced enough driver and proper conditions.
Murderers Are Rapists: Some are, and the inversion of the trope, that all rapists are potential murderers, is definitely true as well.
National Geographic Nudity: Nudists for the most part (most nudists/naturists frown upon the sexualization of nudity and there are explicit "no sexual touching in public/of someone you aren't already in a relationship with" and "no cameras" rules at a lot of retreats and the like, because the subculture is about body acceptance, not about hookups or making porn), and some cultures do practice this.
Naughty by Night: A lot of professional people and the like, although this is becoming far less common with the advent of ways to capture someone in "inappropriate" behavior and blackmail or shame them for it, possibly getting them fired or shunned.
Naval Blockade: A real-life military tactic that has been used in several wars.
Nay-Theist: There are some people that, while they believe a god or gods exist, also believe him/her/it/they are evil or disinterested in humanity, or have some other reason to feel disgust or distaste toward him/her/it/them. Also common in some people who don't believe a specific god exists, but if that god existed, would hate him/her/it for his/her/its percieved cruelty or unfairness.
Near-Rape Experience: Has happened with some people who sincerely didn't realize that "no" actually meant "no" or who were drunk/high, but then realized in time that that no DID mean "no" or where being pushed away or screamed at got through the altered state. Why a Safe Word is a good idea, especially if you're doing certain forms of fetish play or if you or your partner or both of you are from a culture where "no" can mean other things than "STOP RIGHT NOW." Also, if someone was going to have sex with a person, but realised in the process that they were too drunk to consent/were passed out.
Neat Freak: The more extreme may have obsessive-compulsive disorder, and most of us have met someone like this.
Necessarily Evil: There have been a few cases of people controlling organized-crime syndicates and the like and yet being relatively benevolent and relatively harmless to outsiders while being a bit less so on the inside to keep control from those who are more rashly violent/don't see crime as strictly business/etcetera.
Peacekeeping occupations by soldiers as mentioned earlier under Dark Shepherd. Obviously it's not an ideal situation, it is often very restrictive of freedom and can result in a Police State or in severe blowback, it can easily become a dictatorship or worse itself, and the best it can do is policing and its only way to truly, decisively intervene is "stop killing each other or we'll kill you both." That said, it is often unfortunately the only way to stop incipient civil war, internal genocides, and similarly extreme problems.
Dictators and tyrants are occasionally this, which has actually caused history to look far more kindly upon Josip Broz Tito and even Saddam Hussein - the dictatorial rule and brutality served the same role military peacekeeping would, in uniting very different peoples as one nation, or at least keeping a tight lid on interethnic and intersectarian tensions. The problem is the cruelty and brutality to begin with. Also, once the dictator(s) are gone, they, being the real life version of a Load-Bearing Boss, the civil "unity" or "peace" they forged tends to vanish.
States (generally Communist or similar, though the oldest was Japan under the shogun Ieyasu) that imposed atheism/agnosticism officially (or at least strict political secularism), while persecuting religious people of varying religions for being religious or converting/being converted on occasion (and this hits Christians and Muslims the hardest, since both are evangelizing religions, where, say, Buddhism or Judaism generally aren't), did somewhat successfully contain religious extremism and/or prevent sectarian factionalism from intermixing with politics to a major degree, and their actions make more sense when viewed in the context that often, at least with most converting/evangelizing sects, religion and political takeovers/political violence are often intertwined. Whether it was at too high of a cost (the deaths and persecutions that did occur, often just to make a point of "don't do this") is very much something on which your mileage may vary.
Nepotism: "It's not what you know, it's who you know" is definitely Truth in Television: those with relatives in business or government or the arts or whatever tend to get "brought in" by those relatives, and then promoted, often beyond their skill and competence.
Neutrality Backlash: The problem with being neutral in many conflicts is that both sides will either assume you to be on the other side or will hate you for not believing in/representing/supporting their side completely. Neutrality is also often (sometimes unfairly) seen as waffling or self-interest or seeking to sell out to the highest bidder.
Never Bareheaded: Religious or cultural reasons sometimes require the constant covering of the head. Islam is most known for this, but there are some Jews and Christians that adhere to various forms of it too.
Never Bring A Knife To A Fistfight: Dependent on the fight and skill of the fighters, but there have been instances where unarmed or poorly armed (or unarmed until they grabbed the weapons of their opponents) fighters have overwhelmingly defeated an armed opponent. It's not very common (especially with guns) but especially at close range, once the element of surprise has been lost, it's very possible for a nervous or unskilled person with a knife or even a gun to get their ass handed to them by someone who is unarmed but is in position to inflict one of the Choke Holds or a Groin Attack, and/or who can wrestle the weapon away.
Never Gets Drunk: Ironically, some types of The Alcoholic fall under this - their alcohol tolerance is so high that what would kill a non-alcoholic barely gets them buzzed. Other people may be, technically, drunk, but they don't tend to show outward, extravagant signs of being drunk: they're not, for example, loud and boisterous or angry or sad or...anything but quiet and calm.
Never Live It Down: There are some things that people will not forget OR forgive - and some of them, especially via the internet, can be incredibly petty.
Never My Fault: Sociopaths love this one, as do narcissists, and it's a part of pretty much any abusive interaction or relationship, especially paired with Why Did You Make Me Hit You?. It's one of the more popular Dirty Social Tricks among all of those groups, and one of the Dirty Social Tricks that if you notice is in play, the only way to get around it is to document the situation objectively (via recording devices) until you can leave, preferably as soon as possible. If it's paired with Why Did You Make Me Hit You?, just leave now, no matter how difficult doing so is - your own mental and physical health are at stake.
Never Recycle a Building: Sometimes for historic value, sometimes because builders are lazy and architects want their plan used. There's also the unfortunate case of, say, a building that was clearly home to a Jack-in-the-Box now saying "Wendy's" on the front, which just looks plain silly.
No Can Opener: Can openers weren't even invented until several decades after tin cans. Before then, consumers were instructed (according to The Other Wiki) to use a hammer and chisel.
Nocturnal Emission: Happens mainly between 13 and 25, but for both sexes its a mere two percent. Nocturnal erections, on the other hand, are normal for all men well into their 40s or 50s, not necessarily a result of what is going on in the dream, and their absence generally means that something is wrong with blood flow to the genital region and possibly elsewhere.
No Fair Cheating: Getting caught cheating anywhere from elementary school to postgraduate education, on taxes as an individual (corporations can get away with it, generally), in professional athletics, and especially in a relationship that wasn't established as open or polyamorous can have extremely negative consequences.
No Fame, No Wealth, No Service: Members-only or restricted entry clubs, restaurants, and other venues. Anything that will only provide service to celebrities.
No Good Deed Goes Unpunished: Be careful when reporting crimes, lest you be accused of committing them, or being a whistleblower on bad conduct because you could get punished for the leak rather than anyone even caring about the misconduct. People can also get punished (e.g. fired from a job or given a detention/suspension from school) if they attempt to do something like intervene in a fight or stand up to someone who's mistreating someone else, like a bully. Sometimes people don't intervene in emergencies for this very reason, because they fear that the person in need (or someone else with a vested interest, like a property owner) might sue them if any damage is caused in the act of helping.
No Indoor Voice: Sometimes it's a result of physical or mental disabilities, but one non-disability variant is that many singers or public speakers, especially those who have had to work without audio enhancing equipment so they can have a quiet voice, develop a distinct loudness to their voice in a combination of thickened/stronger vocal cords and habit. People in these professions are also often occasionally advised if their voice is naturally loud to not take effort to speak more quietly - maintaining a soft, whispery, airy voice actually damages the vocal cords almost as much as screaming does, so a slightly louder than "normal" speaking voice (if it is usual for that person without effort) rather than a "pleasing" one is healthier for someone who does have to scream or shout on a regular basis.
Nominal Hero: Quite a few people who do heroic things in real life only do them because it is all in a day's work or because they wanted attention or money or fame.
Non Sequitur: "No, I don't think the mailbox needs painting."
Noodle Incident: Most of us have had at least one... or will have at least one...
No One Could Survive That: Occasionally said verbatim in the aftermath of disasters. And occasionally people somehow do survive it.
No OSHA Compliance: There are many places in the world with few to no workplace safety regulations, and even in the countries that do have workplace safety rules and means to enforce them, sometimes enforcement is lax or no one really cares.
No Periods, Period: People on some forms of birth control, who are transgender (MT Fs who didn't have the required organs to menstruate in the first place, or FT Ms who are on birth control or have achieved satisfactory testosterone levels and/or had the female reproductive system surgically removed), who have had hysterectomies, or who suffer from illnesses of the female reproductive system.
Nose Shove: Small children will indeed put anything small enough into any accessible opening on themselves or other things if not supervised.
No Sex Allowed: Some religious systems that impose celibacy or heavily restrict sex. Also applies to minors for obvious reasons.
No Social Skills: Asperger's and other autism spectrum conditions. Agoraphobia. Extreme shyness/social anxiety. Isolation from any social contact at all (even the internet) for long periods of time such as hermit living or religious solitary living or solitary confinement in prison.
Nostalgia Filter: Almost omnipresent with people of certain ages believing things were better in the past even when they weren't.
No Such Thing as Wizard Jesus: Just go ahead and try to tell some Christians that there are parallels to Christian belief in many other religions and beliefs and mythologies, and add to it that maybe it was accomplished the same way. Those flames will burn bright for a long time...
No Swastikas: Carries the force of law in much of Europe where the sale or trade of Nazi memorabilia, the glorification and sometimes display of Nazi symbols, etcetera is against the law. Elsewhere, there is no way to display a swastika without it being seen as support/sympathy for Naziism.
Nothing Is Scarier: Absolute silence is often one of the most frightening experiences, because it can mean a place is absolutely devoid of living human activity. The absolute silence after a power outage is close, as it implies that power and communications have been knocked out by something. And one of the scariest silences is when severe weather suddenly quiets to a perfect stillness, because that is often a sign of an impending tornado.
Not Me This Time: People who have been accused of some crimes or some offenses often are the first blamed when something similar happens. An example of this that doesn't involve crime can be seen when drama starts on a community and someone is dogpiled for starting the drama/Flame War "again" even if they had a legitimate point.
Not My Driver: How many kidnappings have occurred, and why schools/teachers are often instructed not to let anyone pick up a kid if they're claiming to be sent from the parent without some form of verification from the parent.
Not So Stoic: It's actually fairly common for people with anxiety or clinical depression to purposely adopt stoic fašades as a coping mechanism, since they have to constantly work to keep their emotions in check. When the fašade breaks (as fašades tend to do), it can be a pretty big shock for others, who only know such people as The Stoic.
Nouveau Riche: Many people who get rich, at least at the beginning.
No Woman's Land: Unfortunately, there's still places in the world where being female instantly makes someone a second or third class citizen if even that.
No, You Hang Up First: A manifestation of the "sunk cost" fallacy in that someone who is speaking in such a conversation will want to stay on as long as they can even if it would be better to stop talking.
Oblivious to Love: Yes, some people can't recognize when someone is in love with them, even if the person actively tells them so or practically throws themselves at them. Often happens with very shy people (who may invoke I Can't Believe a Guy Like You Would Notice Me to deny that someone could ever love them) or with people highly distracted by other things.
Obnoxious In-Laws: Unfortunately, for some, and often as a result of a societal construct of parents hating their children's partners.
Obsessed with Food: Gourmets, "foodies," and some cooks/chefs on one side, anorexics and bulimics and others with eating disorders on the other.
Obstructive Bureaucrat: One reason people dislike and distrust government and unfortunately the face of it many people are very likely to meet at one point or another.
Obviously Evil: There have been quite a few examples. None of which shall be discussed, but there have been.
Obvious Rule Patch: Many laws and regulations. Building codes and health codes are a very good example: most of them have been written by shown potential for disastrous failures and the attempts to prevent them from happening or happening again.
Offered the Crown: Again, by default, any trope concerning royalty is automatically TIT
Off the Rails: Anyone who has ever been involved in any type of improv or roleplaying situation knows how easily this can happen. For live musicians or theatre performers doing non-improv, one of the most understated and important parts of performing is making sure it doesn't visibly happen and if it does, it's handled somewhat humorously.
Off the Table: Some offers are very much one-time offers, especially if one is rude or offensive to the person making them in their rejection.
Oh Crap: All of us have had such moments to some degree. In a literal sense, if you've ever flushed your toilet and seen the water start rising in the bowl...
Older than They Look: Celebrites, models, musicians. Anyone who was lucky enough to win the genetic lottery to avoid traits like early hair loss or graying, and anyone who was lucky enough and smart enough to avoid sun exposure and tobacco exposure from an early age.
Old Flame Fizzle: Because sometimes whomever people meet first or a long time ago is not someone they will love years later.
Old Friend, New Gender: People change over time. That fat nerd you knew? Dropped the weight and became hot. That man you knew? Was actually a woman, or vice versa. The Hollywood Atheist you remembered from college? Is now a devout churchgoer. Can be very difficult, especially if the changes make your relationship now incompatible.
Old Man Marrying A Child: In most modern cultures this is considered pedophilia - but in other cultures and in historical cultures, it was not seen as such and was actually endorsed.
Old Master: Anyone who is still practising their art or profession in their senior years, not just martial arts, is likely to be one of the best in the world.
Old Shame: Everybody's got something they're embarrassed about.
Omniglot: Some do exist, and Google Translate and other machine translation is slowly improving by the day to help those who don't have natural omniglot abilities to understand in the absence of translators.
One Drink Will Kill the Baby: Alcohol is toxic to humans, but for most adult humans, in amounts well below the LD50, the "poisoning" produces enjoyable effects and no lasting damage. Once one gets near the LD50, one can die or be permanently sickened. In pregnant women and in children, the damage can occur much faster - causing fetal alcohol syndrome or death. Same for pets. And chronic overuse even below the LD50 can lead to lasting and permanent illness or even cancer.
One-Hit Wonder: Quite a few musicians and musical acts. Happens to writers as well, where one of their books or series is a huge success but others, not so much.
One-Man Army: Under the right circumstances, this is completely possible. Sometimes inverted in One Man Armies going down against sufficient numbers or overwhelming volumes of fire, and almost impossible in a situation with air support. An excellent real life example would be MSG Gary Gordon and SFC Randy Shughart at the Battle of Mogadishu in 1993. Technically a Two Man Army, but still qualify.
Simo Hńyhń, a Finnish sniper during Finlands winter war (1939-1940) he scored 505 confirmed kills & 542 unconfirmed. Also 200+ kills with submachine gun. Remarkably, all of Hńyhń's kills were accomplished in fewer than 100 days with a very limited amount of daylight per day. Nicknamed "Valkoinen Kuolema" in Finnish, which translates to "White Death".
One Mario Limit: Some names are so famous or so notorious that naming after them is not a good idea. There is a reason that people with names similar to that of a dictator or a Serial Killer or a terrorist will often change them.
One of the Boys: Quite a few women. And some people who transition gender to male have to do this in situations that require the "real life test" for transition.
One of the Kids: Happens to parents sometimes or to others who spend most of their time around children (teachers of young children, daycare workers).
One Phone Call: If arrested, you might get One Phone Call, none at all, or as many as you want. Unfortunately, your personal phone will be taken away from you (good luck if you relied on it for memory) and the call will be monitored. (This trope is why it is a good idea to, if you're ever in a situation where you fear arrest may be imminent, to write the number of your defense lawyer/of the person you know can and will help you on a place like your foot or the inside of your arm.)
Only Cool People Matter: Some scenes and subcultures evoke this. It's often the death knell of the scene/movement/subculture, though, because whereas the coolest (for whatever reason cool is percieved) may drive the scene/movement/subculture, what is "cool" in it can change in the blink of an eye (e.g. one day, being a Troll is cool, and the next, after a horrific incident of the trolls going too far, being a carebear is the coolest), and also, the slightly less cool and even "uncool" people within it are what gives it lasting depth and following.
A common joke relates to a shirt that says something like "(Bomb Squad, gas company, nuclear emergency technician, tornado chaser)" and then adds "If you see me running, JOIN ME."
People tend to freak out if someone close to them suddenly starts acting differently. This will often prompt a question of, "Did something happen?" or, in the case of romantic couples, may lead them to suspect their partner having an affair.
Ordered Apology: Parents use this to children as one of their methods of teaching them to accept responsibility and make up for something they've done wrong. Also done with teachers to students. The downside of this, however, is that the person is only apologizing because they have to, not because it's coming from the heart, which explains why some apologies are half-hearted.
Ordered to Cheat: Has happened to professional athletes and to students alike, as well as in the accounting departments of certain financial corporations and the like.
Orifice Invasion: There are some animals and insects that do this, both to animals and humans. Parasitic worms are one of them.
Ornamental Weapon: They exist. Often crosses over with Bling Bling Bang for guns, and Japanese swordsmiths in the days of samurai often made swords both this and functional.
Outlaw Couple: Because getting involved with a criminal often draws one into crime in one way, and in another, because criminals tend to interact with other criminals.
Outside-Context Villain: Some real life civilizations acted as such to others. One example would be the Spanish. When the Spanish invaded America, many indigenous populations were either at war with one another, or faced a larger threat (such as the Aztecs for many). Then came the Spanish, with both animals, weapons, and ailments they had never encountered before, wiping many civilizations out, or pushing them to the brink.
Out of the Inferno: Firefighters have done this on occasion, sometimes with people they have rescued.
Out with a Bang: Yes, there are some sexual practices that can kill you (breathplay and autoerotic asphyxiation being the most likely, along with some forms of bondage). If you're in bad enough health or are just unlucky, sex itself can do so. Also true for some animal species.
Overheating: Why your computer needs fans or some kind of a cooling system, especially the more you push it to do. Why ignoring the "check engine" warning or the temperature gauge on your vehicle is never a good idea. Can happen to human beings also, usually during heat waves, under the influence of drugs that affect the brain's temperature regulation - from alcohol to MDMA to neuroleptic drugs - or who have some forms of brain damage that do the same.