OOC stands for Out Of Character. When you are logged into the MUSH, OOC refers to any player-to-player interactions outside of RP. These can include OOC pages, chatting in the Lounge, @mail messages, and so on.
OOC is also used to describe any information which is available to a player, but not to their character/s; for example, a player might be aware of ________. GarouMUSH players are expected to understand and respect the line between IC and OOC information, and play the game accordingly.
See also: Wikipedia's definition of OOC.