Is your character in a fandom, or our they for a original book that your creating? This is extremely important for the development of your OC!

If your OC is for a Fanfiction....

Study the fandom you'll be using for your OC. You wanna make your OC fit in with the fandom your gonna place him/her in, yet they should stand out at the same time. I suggest studying the outfits, hairstyle, body structure, superpowers (if the fandom has any) and lifestyle of the fandom and go from there; is it Western? Japanese (anime)? Futuristic? There's a lot of questions you can ask yourself about the fandom you wanna use for your OC.

If your OC is for a original book your making.....

This is gonna be a bit harder than making a fanfiction, cause you are the creator of this book. You and only you decide what the world and society of this book is gonna be like. But this can also be a good thing, cause now your not confined to a fandom to make your OC. You could make the book (and OC), be anything, and you can do anything you want with the OC! Definitely think of what you wanna write in your book before making your OC; is the book gonna be set in a elven/magical land, filled with dragons and monsters? Or is the book all about robots and the Apocalypse? You'll need to ask yourself more questions than you would if you were making a fanfiction OC, but it'll definitely be great for your story!

P.S. Never be afraid to ask questions about your OC and their book/Fanfiction. That's how you develope your OC; asking questions, then getting the answers through the internet or other means.