sometimes it can work for a character to be OOC, like maybe in something AU, but most of the time it just gets confusing, and not satisfactory as a reader.
Part of the pleasure of fan fic is that you go into a story already knowing the characters. They may do things that are new and surprising, and that's the originality of the author's work, but the characters should generally be who we're expecting,and their world should generally be what we're familiar with and is portrayed in the show or wherever they're from
It's also part of the challenge of Fan Fic writing to capture the reality of the characters and the world
