OC is an acronym that can stand for either original content or original character. While the former is a rare (yet appreciated) occurrence, the latter is frowned upon throughout most of the chanverse. The idea of original characters has also spawned the idea of UCs, unoriginal characters. Though one would think that the idea of an "unoriginal character" would be something negative, UCs are often preferred to OCs.
Throughout every fandom, there are always fans that decide to create their own character based on the ones in the media that they admire. The results often vary from passable to cancer. More often than not, it turns out to be the latter. An original character that is considered to be overpowered is dubbed a mary sue. Because of this, it is often frowned upon to have an original character, except for on a select few chans.
Original content is something that the chanverse needs to see more of. Most of the time, images on chans are just reposts, coining the phrase "every day is repost day". However, once in a blue moon, a user will make something completely new and original. This is encouraged throughout most chans. Though original content can turn out badly, it is a seldom occurrence that the bad stuff gets posted, and even if it does, it will never get reposted.
In order to counter the onslaught of original characters, some users have adopted "unoriginal characters", already established characters from existing media. This has also adopted the name "avatarfagging", though the term can be used in the case of both UCs and OCs. The reason users are prompted to do this is that already established characters have been proven to work, therefore eliminating the possibility of a character being categorized as unoriginal or a mary sue. While original characters are common on the internet, unoriginal characters are more prevalent on chans.