- Awesome Music:
- Broken Base: The ending. It went for a Gainax Ending and left a lot of questions unanswered and that's split fans. Some view it as suitably strange like the rest of the series and the whole sequence being a play made sense within the strange narrative of the themes of the show. Others view it as a grand scale equivalent to the All Just a Dream trope and made most of the questions and mysteries raised by the series ultimately meaningless.
- Complete Monster:
- Alan Gabriel is a terrorist, Serial Killer and thrill-seeking psychopath who leads the group The Union. Obsessed with hurting others, Gabriel gleefully carries out his group's assassination missions, especially targeting sentient cyborgs to hurt them the most. Regularly organizing destructive terrorist attacks on Paradigm City, Gabriel betrays The Union for Alex Rosewater, killing his former comrades before hunting down heroine robot Dorothy and trying to torture her to death to mock the hero Roger Smith with it. Growing bored with working for Rosewater, Gabriel is given his own giant robot, a Megadeus, and proceeds to attack Paradigm City, intending on slaughtering and massacring everyone he can.
- Eugene Grant, from "Missing Cat", is a truly despicable Mad Scientist convinced that he is a god due to his recent discoveries in transmutation. To constantly improve his alchemic transmutations, Grant kidnaps innocents by the dozens and transforms them into pain-stricken abominations that he keeps alive and on display for no particular reason. When many of his old partners abandon him because of his depravity, Grant has one mauled by one of his monsters, and kidnaps two of his former compatriots' son Roy before turning the boy into a cat, following this up by murdering said couple in front of their son. Grant proceeds to transform Roy into a monstrous chimera, then sics it on Roger Smith while he holds Smith's partner Dorothy at gunpoint, ordering Roy to destroy first Smith, then all else in his path. Despite being a Villain of the Week, Eugene Grant handily stood out as the most wicked and irredeemable monster in the series alongside Alan Gabriel.
- Crazy Awesome: Schwarzwald lost his mind before the series began but he's still a hammy, quick thinking, powerhouse who manages to be a serious threat with his own Megadeus.
- Cult Classic: While not nearly as popular as manga that would come out in the following decade, it has a cult following. It being the last fully hand-drawn anime series before its second season as well as its Westernized style gives it a unique identity in the anime world. It also helps that Cartoon Network loved the show so much that they created a second season for it.
- Fanfic Fuel: At the end of the first season, only terrestrial Big O and flying Big Duo were revealed. So what about the third Big? Is it aquatic? Is it Big Solo? This gets answered in the second season, but since there are Bigs for 0, 2, and 4, now the question is what about 1 and 3?
- Fan-Preferred Couple: Roger/Dorothy. In the English dub, this is made even more textual by Roger in season 2 by saying to a then deactivated Dorothy that if they had met in another time and another place and if Dorothy were human, then it would have been very likely that they would have fallen in love.
- Dorothy's dub voice actress outright states Dorothy is in love with Roger.
- Genius Bonus: The trippy opening sequence features newspaper clippings in the background, announcing veridical events such as the death of the 1950s jazz musician, Lester Young.
- Germans Love David Hasselhoff: The series wasn't very popular in its native Japan. America, on the other hand, liked it so much it got a second season designed specifically with Western fans in mind. Appropriately, this is exactly what the designer of the series wanted.
- Hilarious in Hindsight: Given a lot of similarities and homages to Batman, this happened a bit...
- Alan Oppenheimer voicing Norman is Season 2, who as noted on the other pages, is an Expy of Alfred. Oppenheimer went on to voice Alfred in the movie Superman/Batman: Public Enemies.
- The series has often been described as "Batman with mechs". In Batman: The Brave and the Bold and Batman Unlimited: Mech vs. Mutants, Batman himself does indeed have a mech. And then even later, in the comics, the mantle of Batman was passed to Commissioner Gordon...who operates in a mech suit.
- Roger Smith is an Expy of Batman. The person who voices Batman in Batman: Arkham Origins? Roger Smith.
- And then he voices him again in the Batman Unlimited films and webseries and Batman Ninja.
- In The LEGO Batman Movie, Batman referred to Barbara Gordon as "B Go."
- One of the more Mind Screw-y reveals toward the end implies that Paradigm City is some sort of a simulated reality and there are other citizens that are actually robots aside from the androids, but were presented as organic beings in the story. An episode of Justice League Action called "System Error" has copies of the Justice League members Batman, Superman, Wonder Woman, Cyborg, and Booster Gold discover that they're Ridiculously Human Robots made by Darkseid, programmed to think and act like the originals, that are used as test subjects for coming up with strategies to be used against the real ones. This has Gone Horribly Right and they were trashing their creator's Pocket Dimension for the originals' sake immediately afterwards. Just like in The Big O's Grand Finale, their fake Metropolis was collapsing around them, until the real Batman boom tubed them to safety.
- Memetic Mutation: It's anime Batman!
- Narm: "I'm one of the tomatoes!"
- Narm Charm: The same line to some people.
- Nightmare Retardant: Schwarzwald is significantly less scary/intimidating when he looks uncannily like Getter 2 with all those bandages.
- Older Than They Think: This may have a Fan Nickname of "Batman with mechs" but prior to this, Bruce used robots called Bat-Knights extensively in the 1996 Kingdom Come Elseworld to patrol Gotham remotely, which were then used to wage war just like the mass-produced Bigs, after his body gave out to years of physical strain.
- Suspiciously Similar Song: The opening theme sounds suspiciously similar to Queen's Flash Gordon theme. To the point that when Bandai remastered the series on DVD, they had to replace it with a new song. Meanwile, the second season's opening, "Respect," never even made it to the first DVD release - it was almost a note-for-note copy of Gerry Anderson's "UFO." Both openings ended up airing on Adult Swim.
- Too Cool to Live: Schwarzwald, the only Megadeus pilot that ever gave Roger any real trouble. And the only proper Megadeus pilot other than Roger. Alan was deemed unworthy (the message was "ye guilty" instead of the usual "ye not guilty") and eaten by the revived Big Duo, and Alex's Big Fau shut down on its own accord ("ye not" instead of "ye not guilty", thus rejecting him as a Dominus) before 'merging' physically with him.
- Villain Decay: The anime version of Beck, though considering many people found his Crazy Awesome Laughably Evil to be amusing as hell, it's not necessarily a bad thing.
YMMV / The Big O