A SubTrope of CanonDefilement.

In DerivativeWorks (especially FanFiction), this term means that somebody is acting largely against his or her established personality. How and why this occurs has a pretty wide range:
* Perhaps the writer simply doesn't understand what he's writing about.
* Perhaps he is applying his own AlternateCharacterInterpretation to them (of which, both PossessionSue, HoYay, FoeYay, and DieForOurShip contributes towards).
* Perhaps he's just writing a TransplantedCharacterFic. Regardless, it's usually frowned upon to post stuff not labeled as such, with the term becoming quite negative if it gets applied to a story by other people.
* If the FanFic is [[SturgeonsLaw of the 10%]], this is likely because [[OoCisSeriousBusiness something in the story's backstory or plotline made them act this way]].

Generally viewed as a very negative trait (if a fanfic gets [[CharacterizationTags tagged]] as "OOC", it's usually not a good sign, though writers will often be honest enough to slap the tag on themselves up front).

Compare CharacterDerailment, which is this applied to canon, and OOCIsSeriousBusiness in situations where characters are noted as acting out of the ordinary in particularly stressful circumstances. See also OutOfCharacterMoment. DracoInLeatherPants, RonTheDeathEater and {{Wimpification}} can be considered subtropes.

Not to be confused with InAndOutOfCharacter. In {{Role Playing Game}}s, it is sometimes necessary to make a distinction between when a player is ''In Character'' and ''Out Of Character'', to know if the person is speaking as the character or as the player. A player who uses Out of Character information (such as the presence of goblins in a room ahead) to make an in-character choice is said to be [[{{Metagame}} MetaGaming]], which most dungeon-masters severely frown upon.