As an adj.
Relating to or characterized by a tendency to dwell on unwanted thoughts or ideas or perform certain repetitious rituals, especially as a defense against anxiety from unconscious conflicts: obsessive-compulsive behavior.
As a n.
An obsessive-compulsive person.
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.