It means original character, or fã character. People create them for fantoms and such. I have a hetalia - axis powers one, Oregon, along with a bunch of people from our RP.
OC= Original Character I have a few, one of them that will probably never see that fanfiction world (I do not publicar my Sopranos fics) is Sonia Moltisanti...the mais level headed twin sister of mafia man Christopher Moltisanti. (TMI I know)