I have never kept Red terrors myself but I know that they can be extremely aggressive hence the name. If I'd never kept them before I wouldn't risk putting one in with my Oscars, I think there could be trouble. One of my cichlid books talks about red terrors, I quote some of what is said
this is the type of fish that when it's defending its young, will leap out of the water at a hand that is placed over the surface of where the fry are located
Maybe just keeping one will cut out the aggression, but I wouldn't like to guarantee that.