I'll give most cringe-ass OCs a pass since chances are that they are probably made by children or teenagers basically so it's all fun and games. So I'll just go ahead and blanket-wave a lot of X-sonas off the board with that. You know, the usual stuff; sad back stories, edgy backstories, non-sensible ones. For a thread like this a lot of them just sort of turn into cannon fodder and don't really mean or do much of anything.
But for OCs in a position that suggest someone should have know better: I'd have to say Blackjack from Fallout Equestria: Project Horizons.
For context: Fallout Equestria is at this point a sub-genre of the broader brony fandom. It's centralized around the fan-fic Fallout Equestria, a cross over between Gen 4 My Little Pony and the Fallout series. By and large you can transpose a lot of Fallout's background and even some of the main story aspects of the original Fallout games on Fallout Equestria and you get the idea. America is Equestria, China is a Zebra Empire, The Master is a divine amalgam of three different characters known as the Goddess, The Enclave are the Pegasi, and the third major villain Red Eye is an amalgam of Caesar and Ashur. It's pretty decent as far as fan-fics go, or stories in general; could use some additional editing to just clean shit up. I enjoyed it, still do to a point. Helps that within the story's framework you can read the awkwardness less as mid-level writing and more as a product of the framing.
The result of Fallout Equestria and its success within the fandom though is a lot of side-fics were spun off as people wanted to partake in the same world and FoE's author Kkat embraced that. You get some more very good stories like Pink Eyes and Murky Number 7. And then there's... Project Horizons.
Project Horizons has garnered a lot of fame within the FoE fandom that I don't feel it's necessarily deserved especially having decided to go back to read the fucking thing again to get the complete story a little over a year ago. The break down of Project Horizons being:
Blackjack, is a slacker of a security guard of Stable 99 (Stables being this world's version of Vaults). Stable 99 is a primarily female run Stable where a small stock of stallions are kept for breeding purposes. The number of them never exceeds 20 of either the regular horse, or unicorn. Once a 21st earth pony or unicorn comes to age, the oldest of that race is killed and recycled in the Stable's system. On this premise the story could perhaps be either a femdom fetish fic and no one talks about it openly or Somber, the author makes a half-hearted and effort-star worthy gender role introspection sort of story. However he does not dwell on it for very long because Blackjack finds herself in the center of a 200 year old conspiracy which has been sleeping in 99 for the passed two hundred years and is forced to flee the stable with the oldest normal horse of the stable, P-21 with the mysterious program EC-1101 in her PipBuck (Pip Boy) in order to pull raiders away from her home who are making their way through killing and eating everyone in order to claim the program by command of a mega cyborg raider Deus, commanded by a ghoul named Sanguine.
And some weird fucking shit happens.
The story adapts every single anime and video game trope you can grab and the story can be easily read as Somber's anime-of-the-week watch list because he cribs so many disjointed themes from them, and adapts them so poorly with even a passing understanding of things you can pick out anything from Dragon Ball Z to Full Metal Alchemist and he understands them about as much as any mid-level video essay themes essayist's comment base. To try and fully summarize the entire story would be a chore itself. And I really have to focus on Blackjack here.
Blackjack takes on all the power qualities of a Shonen character, dying two or three times throughout the story only to be revived in steadily more powerful forms. Her first death is where many critics of the story say Somber really should have ended it, but by chapter 32 or whatever he's not even half way through the plot because he has some thirty more to go all of steadily increasing length until many are full novels in their own right. She is brought back to life through the magic of technology, and make into half a cyborg. But this isn't enough either because I swear she fucking dies again and comes back as even more of a cyborg and goes even further. Eventually she is killed for the third time and gets a biological body reset and at the end of the story is again killed and without the intervention of the preceding technology made up on the spot to fulfil the function of simply reviving her spontaneously comes back to life in the prior biological reset body that she before transferred herself out of to reclaim her old robot body.
An early character theme for Blackjack is that she's abusive or soft-abusive to all her friends, having had a history of raping P-21 back in the Stable and for the first twenty-ish chapters having to deal with and come to terms she's actually in the wrong and apologizes for it. This is supposed to be an episode of character development for her where she learns to overcome her past and patch things up redeem herself to P-21 particularly after her refusal to honestly admit to and express forgiveness leads to a suicide attempt by him. However the longer the story goes on the less it seems she ever really learned her lesson and keeps going being emotionally abusive to her friends which is simply hand waved away as being "oh you silly pony you lul" and becomes framed less as a tragic flaw but more as a funny character trait. This goes up to the point where she basically rapes P-21 for a second time by coercing him into bed/a three some when he was too drunk to consent and then gets knocked up for it.
Did I mention she also gets P-21's boyfriends killed? She does that. She's directly implicit in or culpable for the gay boy's only love interests being killed and it doesn't really become a problem between the characters.
Throughout the story, because of Blackjack's shonen level importance many of her supporting characters circle about her as mostly being appendages of Blackjack and never give resistance themselves and follow her along without protest, killing much the same as her, and offer complimentary back pats when Somber has to find the need for the tragic breakdown when Blackjack has to have a depressive episode over the lives she takes for flavor reasons, I guess. And even when they do protest, in the end they all go along with it totally fine.
In one such even P-21 says they should release all the slaves of one faction of the story's setting, and rightly so. But despite previous passes by Blackjack being anti-slave, she refuses to because the leader of the faction is technically a cousin several times removed and "a nice person" or something. This is a problem for all one scene in one chapter and never comes up again.
Despite repeated use of the line, "not a smart pony" in reference to Blackjack, she is able to somehow still cut through the thickest of conspiratorial mysteries and histories by sheer existence. She uncovers a 200 hundred year old deep state plot to overthrow the war-time government of Equestria, over lapped with cult shit and coercion between the two sides of the war, and Zebra prophecies and mysticism of the Lovecraftian occult variety. She never has to actively do anything to figure it out besides kill the pre-requesite one-hundred waste landers between each objectives, and neither do her companions despite several being objectively far more intelligent than she. They just stumble into each and every layer and stage of the conspiracy and give all the credit to Blackjack.
And despite attempts by the story to be philosophical over concepts such as ethics and responsibility, it never goes anywhere and Blackjack's response to all this is to become the dumber Socrates and scream "but y tho" while the more intelligent comrades make no comment or are actively as dumbfounded by such riddles and become functionally retarded and help no one in development. But this is less a fault of Blackjack per-say, and more Somber.
Taken at this face value though you might say, "well this seems fairly OK. Not good, perhaps. But not awful. Shonen leveling up aside"
Well a part of the frustration this character gives me and what has cursed my soul to my last day knowing I have wasted 1.8 million words of my time with is: no one says anything. Blackjack is the terminally qt pure waifu who never did anything wrong to the fandom and like in the story no one stops to ask: "is Blackjack really the role model we pretend she is for raping her friends and lusting after minors? No, she is the angelic one!" She's continually received as a pure character, with most everyone being as indifferent to her shenanigans as everyone else in the story was to her. Blackjack got to die a hero, and maintains her pure trad waifu status among Project Horizon's readership.
Where as in comparison: Little Pip, the hero of Fallout Equestria has not had her massacre of Arbu forgotten. Though she killing an entire village had weight in the story and punishment. Blackjack I swear was complicit in destroying several and no one cares because the story doesn't care; it went on as if she was innocent. And the one hard moral decision she makes in the first thirty chapters to shut down the stasis pods of thirty broken and dying two-hundred year old foals is forgotten pretty quickly.
So yeah, death to Blackjack and P-21 is the true messiah.
The truly forsaken one. Everything would have been 200% better in all ways had he been the central figure.
EDIT - To connect things to the OP: did I mention Blackjack has a sword? Yeah. She has a sword. A very special one. She's a sword and shotty character. Her shotgun typically shoots explosive rounds, and her sword I guess absorbs souls for ??? reasons because it's made of meteorite metal and basically can't be destroyed and cut through most anything. I think she takes on a tank with it. I can't fucking remember. I do remember she uses her cyber legs to yoink off the hatch of a tank though to get inside and yell at its horse-brain powdered computer.
A horse brain that is, to everyone's shock: from the brain pan of Deus the previously mentioned mega cyborg raider she earlier killed in the story by blowing up his head with a grenade or something in the first twenty-ish chapters.
She then uses the sword to - I assume - kill a eldritch nightmare god.
EDIT 2 - So every time I look at the OP I see something comparable Blackjack has done. In the course of the story she goes to the moon, twice. First to confront the Big Bad who wasn't really the Big Bad and get he body back which has the gestating fetuses she conceived with P-21 and also to save the world because the moon had a military base built on it centuries ago with the intent of delivering a world-ending payload by the rebel Equestrian faction for ??? reasons that are revealed to be tied with the eldritch god's influence. Several centuries later after the events of the story because Blackjack is cursed I guess with eternal life or just super slow aging per the conditions of the body she came back alive into she returns to the moon with Little Pip who also has prolonged life through contact with mutatgenic compounds because Somber really likes Blackjack x Little Pip shipping for some damn reason. But anyways, she goes back to the moon to retrieve a friend she abandoned there who is likewise cursed with immortality through being bound to a healing artifact so far corrupted beyond its abilities it can fix death, and so continually revives her from even annihilation; if as a filly. She also crash lands in this second trip and survived, Little Pip does too but as a thoroughly demeaned accessory to it all.
In the time-span of the story she essentially crosses enough territory to have hiked across the country of Equestria. The story in its 1.8 million words of mayhem is only two or three weeks long. Sometimes she manages to cross the entire core region in what is basically a day.
Blackjack throughout the story encounters and fights a variety of Extremely Anime Villains and Characters that range from "cyber pony Deus who is in many ways canonically a traumatized tank, figuratively AND literally", a mutated and colossal prison warden who has incorporated the full prison defense apparatus into his body like some Metal Gear abomination
Additional villains include gods and demigods, the pegasi enclave's military-industrial apparatus (alone, Little Pip had to fight them too but she also called upon and organized an army of her own through the various contacts she made and the reputation her story in-universe got her) and an epileptic bat orgy.
EDIT 3 - Did I also mention she catches one of her underaged companions doing the naughty bumps with an adult character? She does and then tells her she did a good job.