Used in fanfiction when an original character is inserted into an already existing set of characters/fandom. Often shunned due to fear of Gary Stus, hence it is given as a warning.
See also: OFC
I'm terrified to use my OMC because some might think of him as a Gary Stu.
Referring to the almost godlike compassion and restraint that the character Carlisle Cullen exhibits in the Twilight series by Stephenie Meyer.
Bella: OMC Edward, please say you're kidding!
Play on "Oh my God". Fans of the hit CW TV show, Supernatural, have come up with this term after the Season 6 Finale when a character (Castiel) proclaims himself as God.
Girl2: OMC, it was gorgeous.