Out-of-character (OOC) is used in roleplaying. In this case, OOC refers to things said by the author of the message, rather than by the character(s) that author is playing.
In game, OOC writing is often denoted by closing the text in double parenthesis. ((This would be an out-of-character statement in the game.))
/say Hello, my name is Bob. ((Actually, my character's name isn't Bob, but he's lying.))
Variations such as double brackets, lesser-than and greater-than signs, and double dashes closing the text are also practiced.