I think they might be too aggressive. Is your oscar very aggressive? When i had one it was peaceful and shy, couldnt stand up to fish its own size if they were aggressive. Probably would have worked with large peaceful fish.
Well I had a green terror and now have a Jack dempsey. My green terror was extremly peaceful! Not a terror by any means. I got it some feeder guppies once and it wouldnt even eat them. They bred in the tank. Although my jack dempsey hides alot( I just got him though) when he comes out he is stunning. He is real mean. He is about 4" and tears 2" goldfish to pieces. The meanest fish I have ever kept though is my oscar. Never hid either and he tore any thing to peices. He was so mean.