Definition of "OC"

One point that almost always seems to be overlooked every time the Mary Sue issue comes up is that #*not* all original characters are Mary Sues. Although there are those who think OCs should be kept out of fanfic, it is possible and acceptable to write OCs into your stories without deifying them.

1999 October 28, Laura Taylor, “Re: Voyager Mary Sue Litmus Test”, in alt.startrek.creative‎ (Usenet)

Too often an OC tends to slip into a Mary Sue mould, but if it's well written, I have no problems with an OC as a protagonist of a fic.

2005 February 3, Don Sample, “Re: Are OC's the kiss-of-death in BtVS fanfics?”, in alt.tv.buffy-v-slayer.creative‎ (Usenet)

For instance, one fan might create an OC or Original Character that does not exist in the primary media canon. Then, with that fan's permission (it is considered good form to request permission from the OC creator; however, this does not always happen) another fan may incorporate that OC into his or her texts.

2008, Rebecca W. Black, Adolescents and Online Fan Fiction, Peter Lang, published 2008, page 40