in my honest opinion, it's just fan fiction and if there's an OC in a story, then fine by me. It does add some freshness to a story and if the original characters are still in the story and continue to be in the story until the end, then great.
If a story was mainly centered around a certain character throughout the whole thing, then, my honest opinion, that would get kind of boring after some time. If it was centered around all the characters, plus an OC, it wouldn't get so boring, and it could make the story even better.