It has been about 10 years since I first began keeping Oscar Fish. I fell in love with these personable cichlids after seeing them at a friend's house and I have learned a lot about them since that time. Through trial and error, I have discovered what species seem to work well with Oscars. First of all, perhaps I should point out what species will never work well:
I have heard of people having success with putting African Cichlids in with Oscars, but I would never try it. The water parameters required by the two species are completely different and the aggressive nature of African cichlids could only stress the Oscars. I imagine it is possible it could work, but neither species would thrive or live up to their potential.
Because Oscars are South American Cichlids, people often assume they mix well with other New world cichlids. I can tell you from experience, your chances are 50/50 at best with some of the more aggressive species like Jack Dempseys, Green Terrors or Red Terrors.
I have personally had great success with the following species:
My current Oscar setup has 6 Red Tiger Oscars, 2 Albino Oscars, 3 Firemouth Meekis, a Pleco and 2 pimelodus maculatus “spotted catfish”.