sorry for a late response- i'm the only person running this blog, so comments sometimes get overlooked...
Any and ALL OC's (mainstream or not) Belong in the Original folder.
IF the art piece is a character from the show plus an OC, feel free to also include it in another category. In The case of Inuyasha and Danny Phantom, you could submit in the Mainstream folder. An OC for something lesser known such as The Boy Who Fell (great comic btw) would still go in Original, but if in the same piece as a character from the comic can then go in ANY folder relevant (traditional, digital, cosplay collabs etc.)
Thanks for asking!