I believe that you have the best chance to have two fish get along if introduced at a young age, but it is still not guranteed. But on the flip side, every fish has their own personality and its possible to have for example a RD and an O together, but only if personlity permits.
YOu have trial and error with any agressive cichlid, and you wont know until you try it out! Most say Oscars are big old punks but I have seen a few in my day that can handle even an RD if provoked. Os have so much strength that they barely use to their advantage!