I've had an oscar and a green terror together and they lived in peace for quite a long time. But I had them in a 125 at the time and before that they were in a 60 when they were smaller.
I have my oscar in with a singe green terror (because the red zebra i USED to have killed the other one), and 2 JD's, 2 pleco's, and a spotted pictus...
all the cichlids get along just fine.... the only problem i had was the zebra, and the oscar used to get in its way when it would terrorize the other fish.. but the zebra's gone now, and not a single issue.