Hello, Bun&Belle!
Did you say you moved back into the home Oscar was alone in for a while? If so, that is likely related to his new territorial attitude. If he is being mean to Belle I suggest separating them for a little bit and then going through the bonding process again, or she may get hurt. Try approaching him with bits of treats in your hand when you’re near and be by him as much as possible? Oscar will likely start warming up to you again. If he shies away from your hand, try holding it away from him (not too close to his face, because bunnies have bad close-distance vision) and letting him approach your hand rather than vice versa. If he has no reaction, slowly get closer and if he’s aggressive, do NOT try punishing him with any kind of pain, (which may be something you already knew, but I’m not sure). A squirt with a spray bottle may work, but it never did for Ginny. I repeat, DO NOT HURT HIM. No tap on the nose, so flicking, nothing. He will only meet violence with violence.
Hope this helped!