Mostly either a from of the creator themselves, or someone else they made up and are usually beautiful and/or powerful.
1 (fanfiction) Most OCs wreak stories when they are centered around them, simply because no one cares about a character that someone else made up. Also, OCs are usually paired with whomever the author happens to have a crush on. Usually, OCs are unoriginal, and people get bored of them. OCs as background or semi-main characters in a story can make it good, though, if the author knows how and when to use them.
2 (roleplay) Roleplay with OCs can be fun, especially if it's an all OC forum. but if there are too many OCs on a canon forum, then it can be annoying. Especially if there is a person obsessed with their OC
All of his story's are all about his OC.
If they're were not so many stories with OCs, then good fanfiction would be easier to find.
That story was so good. Sure, it had OCs, but they weren't the main characters and really helped the plot.
There's this one girl in roleplay who's obsessed with her OC, and she ruins it for us all.
Many OCs appear as long-lost family members, past lovers, or exchange students. These background stories provide alibis for the sudden appearance and previous absences of the OC.
OC may also stand for "other character," in this case representing an extremely minor character, introduced or mentioned but not extremely active in an original series.
Ayame is an "other character" from Naruto.