First of all, I'm going to list some obvious things not to do before going into more detail!

Do not:

- Make your oc the most beautiful person ever, it's ok to have pretty oc's and it's ok for them to be absolutely stunning. But please don't make them the most canonically pretty character as those roles tend to go to people like Momo, for example, as she was designed with the intention for her to be beautiful.

- Don't be afraid to experiment. Go all out! Give them wings, a tail, horns, ears, claws, scales, fangs, tentacles etc. The bnha universe is a unique and colourful place where everyone looks different. Don't be afraid to make them stand out. The whole point of an oc is to be original so be original and just have some fun.

~~~~~~~~~~~~~~~~~~~~

Mutation

If you're going to give your oc a quirk that changes how their body looks, go for it! Try everything. Maybe they have a dog quirk that gives them a black nose, ears, paws and a tail. Or maybe they have a tiger quirk that gives them striped skin, claws and fangs! Diversity is honestly so much fun and let's be honest, if you're not enjoying making your oc them you might as well not make them. Mutant quirks aren't just for animals, they can also just be random things. Maybe they have a 10ft long tongue that's strong enough to pull down a building or perhaps your oc permanently has rainbow skin that can be changed at will to camouflage, who knows? Like I said, diversity and experimentation is key so just go for it and play around until your happy.

Appearance

Please, for the love of god, do not make your oc the typical person who's "Blonde with silky hair, has big blue eyes, is skinny and petite and has pale, porcelain skin." That's mind-numbingly boring. Give them lime hair that's styled into two triangle-shaped buns or maybe give them dreadlocks that are wrapped around each other like spirals. It's fine to have an oc with blonde hair and blue eyes but at least make them a little more cool. Maybe that silky blonde hair of theirs floats because of their quirk or maybe one of their eyes is a brighter blue than the other. Anything that makes them just a little more interesting. Try and use colour palettes or colour generators to come up with some cool combinations and definitely use references form the internet!
Now onto their skin, and this is important. There's nothing wrong with pale skin and it's likely your oc is Japanese, so they might have pale skin. However, there is also nothing wrong with switching it up and giving them either darker skin or mutated skin. Rock Lock has black skin but he fits in perfectly, Mina has pink skin but she's treated the same so try out different skin colours every once in a while.
Lastly, don't be scared to mix up body types and flaws. There is no shame in making your oc a little bit curvier or even making them plus size. Of course, there's nothing wrong with being skinny, it's just as normal as being plus size, but occasionally it's good to change things up. When it comes down to 'flaws,' you could give them freckles, imperfect teeth, heterochromia or something like that.