Short for 'Out Of Character'.

1) Not speaking as a character, but as a player - Posts in the OOC Community are the Muns, not the characters, and notes in '((' double brackets or sometimes '[' square brackets are 'OOC notes' between muns.

2) The character is acting different to how they would in canon. A mun calling another mun's character OOC is insulting their playing ability and/or grasp of the canon, but if given as concrit it can be very useful. In fact, sometimes newer players will ask to be told if their characters are OOC, so that they can work on characterisation.