I have owned both oscars and GTs, but never together . . . but the reason I no longer keep either is that it was too difficult to keep anything else with them. I lost my oscar when it tried to eat a pictus catfish (not a good idea, he choked to death) . . . and I had to give up my GT 'cuz it was getting owned by my JD
FWIW, instead of GTs, I prefer blue acara . . . still a good looking fish, and not as much drama in the tank . . .