- Perhaps the writer simply doesn't understand what they're writing about.
- Perhaps they are applying their own Alternate Character Interpretation to them (of which, both Possession Sue, Ho Yay, Foe Yay, and Die for Our Ship contributes towards).
- Perhaps they're just writing a Transplanted Character Fic. 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 of the 10%, this is likely because something in the story's backstory or plotline made them act this way.
Generally viewed as a very negative trait (if a fanfic gets 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 Character Derailment, which is this applied to canon, and OOC Is Serious Business in situations where characters are noted as acting out of the ordinary in particularly stressful circumstances. See also Out-of-Character Moment. Draco in Leather Pants, Ron the Death Eater and Wimpification can be considered subtropes.
Not to be confused with In and Out of Character. In Role Playing Games, 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 MetaGaming, which most dungeon-masters severely frown upon. Also not to be confused with the acronym for "Original Canon Character", another term for an O.C. Stand-in. An Adaptational Skill in a fanfic might be out-of-character if it's highly unlikely that the character would have it (e.g. The Ditz being able to do surgery).