OOC means out-of-character.
The OOC area on XMR contains areas such as the character generation rooms, guest starting rooms, staff areas, etc. This area is considered to be out-of-character and to not really affect gameplay at all. Anything you say or do in these areas reflects you, the player, not your character, as a general rule.
Also, there is an 'ooc' command which allows players who are on the IC grid to chat in an out-of-character manner to work out outcomes of combat scenes, to clarify what people meant in their poses, etc.
Most channel? chatter is considered OOC, though there are a couple such as a chatroom one which is considered IC.
See also IC.