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!
Sometimes used to disguise actual meaning when the company around you would take offense to you saying "old man cock."
As in, you can't interact with a webcam (ie. on Chatroulette) without some old man cock popping up on the screen once in a while.
You know who has the best OMC? Your dad.