OC Fiction 

Stories written around or composed entirely of original characters (OCs). Tends to be a better form of fanfiction than that made up of nothing but canon characters, as it's a lot harder to mess up those characters' personalities. It's highly recommended that anyone trying to get into fanfiction should start out doing OC fiction.
Games such as Warhammer 40,000 and Dungeons and Dragons almost demand that you write OC fiction for them, since there are very few canon characters (none but the gods in D&D, and they can act pretty much how the writer wants them to within a certain boundary). As such, these make excellent starting areas for authors looking to get into OC fiction.
If you're writing OC fiction, make sure that every character you add to the story is taken through the Mary Sue Litmus Test first, unless you want to get flamed. Mary Sues have killed many a good piece of fanfiction with their sickening, unrealistic perfection.
OC fiction often allows readers to submit their own OCs; don't just accept them out of the kindness of your heart. If they're Mary Sues, or don't fit within your story, then don't accept them. Simple as that.

I was really enjoying an OC fiction fanficiton; they'd gotten the character I'd submitted completely correct. But three chapters in, some dumbass submitted a Mary Sue into the story, and the author didn't know how to say no.

OC Fiction by Snowskeeper April 4, 2013