OOC, or Out Of Character, is the term that describes anything that happens outside of the main Midian role-play. Example: If you, the player, knows that the doctor at the MCMC is a cannibal, that is OOC information. Your character may not act on this information unless he or she has found this out through the process of role-play. Or is being nibbled on in the waiting room. If your character displays knowledge that they could not posses, it is considered meta and action may be taken against you by a Game Master or Admin.
OOC is also used to describe chatter, usually marked with double parentheses ((Like this)) or double brackets Like this that separate OOC chat from regular, role-play chat. This allows you to say something to other players, either a joke or a question about the role-play at hand, without disrupting the game.
Example: If you were in a combat situation and were unclear as to if the cannibal doctor is attacking you or your companion, you would say in Local Chat ((Hey, is Dr. Toothsome on top of me or my rather savory, juicy friend?)). Since your character would know the answer, but perhaps you forgot, this is an acceptable way of getting that information to use in the role-play.
OOC chatter should ALWAYS be separated from regular, In Character (IC) chat whether by brackets or (preferably) through an Instant Message to the concerned parties. It should be kept to a minimum in Local Chat to avoid distracting those role-playing around you.