What is OC

It means Original Character. Not to be confused with OOC which means Out Of Character.

tu have OMC and OFC = Original Male Character and Original Female Character, respectively.

For your info, there's a spot here about Severus Snape and Original Female Characters : link
You're welcome to registrarse !

OC = Own Character o Original Character. It's a character the fanfic writer has created himself o herself, not a canon character.