I would like to go on record saying that I believe all OC's are bad.
In terms of appearance, you can have a cool idea, but only if nobody else has done it. Heterochromia is a really cool trait the first time you see it, and then you realize how overdone it is. Any one trait is probably either overused already or trying too hard to make the character a unique snowflake. Or surprisingly often, both.
In terms of backstory, the only acceptable route is to create a character that is basically inconsequential to the universe. They can't interact with canon characters in any meaningful way, and they can't be anyone we should have already heard of.
So by that logic, an example of a good OC would be a donkey named Horatio who lives in Manehattan and lost mobility in his front left leg in an accident while he was working at a ball bearings factory. And in just about any other universe, that kind of character would be perfectly acceptable, but because we're all fans of a happy horse world where anything more than slightly depressing is out-of-place, that's not a particularly good OC either. To make him fit with the universe, we would have to give Horatio back the mobility in his leg. If we do that, he loses the one unique character trait we've given him, and he's just a donkey. Probably a sad donkey because he works in a ball bearings factory. (He could have collected on disability and used the extra time to learn how to sing, but the accident never happened because it's too dark, so he can't sing very well, and he still has to work.)
So, without giving him any supernatural abilities that a donkey wouldn't have, giving him any significance to anything that appears in canon, adding anything too dark or depressing to appear in the show, or making a really boring character, how would we get Horatio to the point where we can call him a "good OC?" Personally, I'd say the only thing to do would be to scrap his entire backstory and make him a brave and daring sea captain with many tales of his heroic exploits aboard his faithful ship, the H.M.S. Ballbearingsfactory. There's just no easy way to make a character without some kind of spectacular role seem interesting.
Now, of course, if we were going to use Horatio for a roleplay of some kind, we could start with an ostensibly bland backstory and add development as the story progressed, but in doing so, we're not creating a good OC so much as working with a banal one until it becomes one. Horatio can't be created as a good character all at once, because character development isn't something you can fit into a personality description. It has to happen within a universe, and it has to involve dynamics between multiple characters, or between a character and their environment. You can write good characters by yourself, of course, but you can't write good characters in a vacuum just by slapping together a few concepts that you might like, because that's not how it works in real life. People are shaped by their surroundings, and characters need to be the same way.
If you take any character from any piece of fiction out of that framework and try to envision them just by themselves, without the rest of the story, they're going to appear to be "too ______," for this reason. Pinkie Pie, by herself, is too manic and hyperactive. Romeo is too naive to exist. Francis Begbie is too homicidal to go through life without being put in jail forever. Atticus Finch is too perfect. Tyler Durden has so many things wrong with him, I don't even. The reason those are good characters has very little to do with the initial concepts that make them up, and at some point, just after those concepts were created, there was no more to them than any OC you might have come up with, and they were, therefore, bad characters. The only way to make a good character is to make a character and then put them somewhere. The best way to make a good character is to make two.