If it's FROM something, then you only have to worry about how accurate the getup is, else, judges would be loose about it. OC can usually get honorable mentions and judges choice at these cons, but usually nothing in placement. If you start adding things that don't belong and still call it an oc, then ppl get skeptical and iffy about the project: what are you really trying to do, your own or someone's developed character.
I'd limit it to no OC if I were a judge of a themed con, else you get infinite costumes(which includes extra gore Ninja Halloween town bleach Street Fighter characters). General theme cons are your ticket for what you want.