Yes Fandom OCs are allowed! Regardless of if you're apping them as OU or AU (aka if they are from their canon world or AU'd so they grew up in Kaisou's Earth) - you'd just mention in their app what canon that they are from and that they are an OC as well.

On the taken list, they will then get listed under their relevant canon (aka XIV WoL's are all listed under Final Fantasy XIV).

Hope that clears it up!