- Perhaps the writer simply doesn't understand what he's writing about.
- Perhaps he is applying his own Alternate Character Interpretation to them (of which, both Possession Sue, Ho Yay, Foe Yay, and Die for Our Ship contributes towards).
- Perhaps he's 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).
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.