I kept oscar and oscars with my aro's for many years without problem..
of course, it really will depend on your fish's temperment.. some oscars can get somewhat aggressive and may not be suitable.. most I have had however were fine..
unfortunately my aro gave my 10+ year oscar a bad hit and burst its swim bladder.. I was sorry to put down that oscar..
(there was some revenge going on - oscar grabbed the last superworm - i only had one left, i threw it in tank for aro but oscar grabbed it.. no more for aro - it got upset and smacked the oscar hard - just once, but it was enough)
If I ever see a full size pure red oscar again, I wont hesitate to put it in my tank.