Get him something so he can hide in it, they like to hide and establish a territory. He is in shock if he doesn't hide. Not good on a fish. And yes he will be very aggressive. That's why they call them "Red Devils".
Not to bash or anything, but what size tank is that? It looks jam packed, when that RD grows up to 10-11 inches it's going to get nasty in that tank, going to be some major fighting between your Oscars and RD. Not to mention the nitrate ppm must be high, and you risk your Oscars getting "Hole In The Head" disease.