what's an OOC?

I think it means "Out Of Character."

Yes, It means out of character :).
But sometimes it's fun to write them out of character XD.

Not to be confused with OMC - Original Male Character - or with OFC - Original Female Character.

If a character is OOC, it can be really good if the story is definitively on the humorous/parodic side. If not, most of the time, it's bad and probabilities are very high that a MarySue or a GaryStu is lurking around.

You may check the article on "Fanfiction" on Wikipedia, to know more about fanfiction terms and lore. I do so sometimes when I get confused over some words and expressions.

"OOC" means "Out of character". That term's usually used for crack fics.