Angst? What Angst?: Eiji has no family, job, or ambitions, but he doesn't really attach any importance to those things so it doesn't bug him. However, he's often haunted by memories of a girl he'd befriended on his travels that he was unable to save.
Even with all the weirdness rampant in #27, one moment stands out: two random women come across the Squid-Jaguar Yummy and smack it around, then disappear before Ankh and Eiji get to it. That's really it — the women are only in the episode because their names contain the kanji for '1000'.
It's more the fact that they beat the crap out of it than they appeared, as it did mistake them for Kamen Riders.
Why does the Yummy belonging to the former Shocker Mook spawn a couple of Oba-san?
Then there's Eiji resurfacing in Brazil at the end of Movie Wars Core, though it leads into a Brick Joke as Chiyoko shows up after a sign at Cous Coussier stated "On Vacation".
Broken Base: Although not as big as the "Who is the secondary of Blade?" debate, there is a debate over if TaJaDol or PuToTyra is OOO's final formnote Super TaToBa is counted as an extra form. The problem comes from the fact that, although in terms of debut order, the latter would fit said definition, not to mention story-wise, it had a role similar to Blade King Form and later, Gaim Kiwami Armsnote both of which are unquestionably seen as the final forms in their respective series, while on the other hand, TaJaDol is seen as the culmination of Eiji and Ankh's relationship, not to mention that it ended up being Eiji's most powerful weapon against the final boss. Much like with the aforementioned Blade secondary rider debate, Toei doesn't take a definitive side, since for every time TaJaDol has been shown alongside other rider final forms, there's also a time where PuToTyra was also shown alongside final forms.
If the Rider Card toy line can be considered canon, it seems that Toei ultimately settled on PuToTyra being Eiji's final form.
Epileptic Trees: Since the beginning, many people began creating theories about the OOO Driver being a Deadly Upgrade, who the Original OOO was, and how it will affect Eiji, with a popular theory being that Eiji would turn into a Greeed. Surprisingly, they're correct about the latter, but it's not the OOO Driver that's doing the conversion. That would be the Core Medals inside Eiji.
Fan-Preferred Couple: Pretty much most of the fans, even those who normally don't care for male/male relationships, rejoice in shipping Eiji and Ankh.
In the advertising for the Cho Den-O Trilogy, Momotaros says of the trilogy's Colorful Theme Naming "Think of a traffic light!" Now we have OOO, who has himself been nicknamed Kamen Rider Traffic Light.
We've had Dr. Maki leaving the Kougami company to form his own faction with the younger-looking Kazari and Lost Ankh in the form of a small boy, not long after the fate of a certain otherTwo and a Half Men was in question.
And the final fight is Eiji with Ankh reduced to Core medals vs. Maki, essentially Two And Bits of a Man.
ShaUTa Combo's theme song title, "Shout Out", is bound to make many a troper chuckle.
Birth's theme "Reverse/Re:birth" sounds quite a call back when it is considered that Kamen Rider Blade's resident Gunslinger Garren's actor sang an ending theme for his series, called rebirth.
The OOO chapter in Movie Wars Core is subtitled Nobunaga's Desire, which may bring to mind one of Koei's strategy game titles, Nobunaga's Ambition.
One of those "weird Japanese inventions" often circulated on the 'net involves a device women can use to outwit stalkers, a handbag that unfolds into a vending machine disguise. Which is exactly what the RideVender does.
You know who else was famously disguised as a vending machine? Dispensor.
The animals that Kazari's Core Medals are based on, Lion, Tiger and Cheetah appeared as Mecha in Juken Sentai Gekiranger.
There was quite a bit of negative reaction over PuToTyra, the purple form, being designated as the "Ultimate Form" over the already-impressive TaJaDor (which was touted as such in Movie Wars Core). Fast forward to the finale, and PuToTyra gets Worfed in favor of TaJaDor as Eiji and Ankh battle the Big Badas one being.
There are three possible reasons for this. One, PuToTyra is the same "type" as Maki, so it probably wasn't as effective as against the desire-based Greeed. Two, a true collaboration between Eiji and Ankh is what the series has been building up to. Three, with PuToTyra out of the list, TaJaDor is the strongest form OOO has, as well as the only form available at the point.
Looks like Toei decided to take the initiative here, by churning out Super TaToBa Form for Movie Wars Megamax. It does reuse TaJaDor's head though.
In The Movie, OOO meets a Tokugawa Shogun, and the primary theme of the show is Desire. Later on, in Fate/Grand Order, there was a special event set in the Tokugawa Ooku and which involved the Tokugawa, and the final boss is a villain who wants to eliminate Desire from the universe by flooding it with her own!
Jerkass Woobie: In #29, when Kazari uses Maki to make a Yummy, it's revealed that his greatest desire is for a hug. Though its pretty much subverted when we find out he's the one who killed his sister because she was getting married and wanted to abandon him. When he remembers this, he doesn't seem all that guilty about it.
It's also no accident that the Yummy created from him hugs women to death. It isn't a case of Does Not Know His Own Strength on the Yummy's part. It's his desire that if someone shows kindness and compassion, they should die before they can be corrupted by the world and taken from him like he felt his sister was.
Kougami; crazy as he is, makes it abundantly clear that he's in control of the situation in #6, forcing Ankh to accept a crooked deal in order for him and Eiji to use the RideVendors located around the city.
Ankh himself isn't too shabby either: In #20, it is revealed that he switched the core medals with cell medals in the case beforehand so that Kazari's apparent victory by stealing it in #19 was foiled.
Moral Event Horizon: Kazari using Gamel and Mezool in an experiment that could quite obviously be fatal and not caring one bit both were killed in the result. In fact, he smiles while he observes his handywork.
If he didn't cross it by locking his assistants in a lab with rampaging Yummy For Science!, Dr. Maki crossed this when he realizes he murdered his own sister and reacts, not with remorse, but by deciding he did the right thing by killing her and now it's time to destroy the world. It's more likely the latter, as before that point, he wasn't beyond redemption. It's this point that it becomes clear he's irredeemable. Made even worse by the tensing of a HeelFace Turn directly before it.
Eiji's father crossed this by exploiting the fact his son was badly wounded and emotionally scarred for life by a war his family's money funded to get votes! How could anyone blame Eiji for walking out on his family after that?
And then there is Kousei Kougami at episode 47 who overacted regarding about the purple core medals stuffs blocking the way. Though considering this is how Kougami usually acts, it becomes more Narm Charm.
In Kamen Rider Double, a representative of Foundation X discusses the possibility of the Core Medals. In Kamen Rider OOO, Foundation X is never mentioned.
However, they are now confirmed to appear in Movie Wars Megamax.
On another topic, OOO's chestplate has been damaged so badly, the Tora Tron Lines were greyed out. It could have been in more scenarios where OOO is in a pinch.
It would have been cool to see Giga Scan combos with other core medals. (In the entire series, only Batta and the Red and Purple ones have been used)
The "Multi-King" form that the Original OOO, Kougami, and the Greeeds try to achieve. Considering how powerful individual combos are (and that Greed!Maki got One-Hit Killed by just both PuToTyra and TaJaDor, respectively), it would've been interesting to see the level of power that form would give out.
Viewer Gender Confusion: Gara has a female live actor and a male voice actor, so this becomes inevitable. Justified, as Gara possesses a woman.
Visual Effects of Awesome: The last stage of the Final Battle is done amazingly well, especially when they fly inside of the Medal Vessel for the final exchange. They really outdid themselves on this one!
The Woobie: If you didn't feel sorry for Gamel while he was wandering around searching for Mezool like a lost puppy, you don't have a heart.
Gamel again in #45 where he's utterly heartbroken by Mezool discarding him once she's fully restored, and eventually ends up with her dying in his arms.
And again with Gamel in the next episode as he dies and tries to give Mezool's last Core Medal a lollipop and hallucinates Mezool accepting the sweet before collapsing into a pile of Medals himself.