I don't have much of an opinion on whether or not they should be allowed at all. But you do allow OCs I think it would be best to stick to just original universe OCs and disallow fandom OCs as a general rule. It could work to only allow fandom OCs from canons that are specifically geared towards OCs (WoW, the Elder Scrolls, table top games, most pre-Dragon Age western RPGs, etc), though.