Yeah OU basically means any character who originally came from another world entirely! CRAU is appable for that as well, since that's a situation of someone world/dimension hopping a few times before landing in Kaisou.

That said, AUs CAN also be CRAUs too -- meaning their previous game memories can be among those they slowly remember, as well as any abilities they gained during the time in that game (for example Arthur who is an AU regained a sensing power he got during a murdergame!). So if you decide you wanna app as an AU, you can still do CRAU stuff with it!