OCs can work if done well imo. There was a Love Hina fanfiction I read a while back with an OC as one of the main characters, and he felt good to read. What an OC needs to do is not try to surpass the main cast, but be able to work with them and the world they are in. If the canon cast can work with the OC and not just the other way around, then that's a good thing.
Also, limits. OCs need limits or people will feel they are overpowered and are Mary Sues. I don't mean simple limits like how a female mary sue is only at her strongest for three minutes in a fight or can only use her power 10 times in her life or something like that, but something that prevents her/him from being the most powerful being in existence. Sometimes a weakness that a weaker character/characters can take advantage of in a fight to overcome them, or a weakness in they're personality that isn't easily overcome.