Well get them young and I mean young. Raise them together and they should do fine. I have actually seen an Oscar, a Jack Dempsey, an Arrowana and a Red Terror kept in the same tank together and they never fought for well over 6 years. They were all the same size when they were put in the tank and grew to very different sizes. The arrowana had to be removed at 3 years old because the tank was to small but the fish never fought.
Is kinda like having a Pit Bull, it is not the breed but the owner, do it right and you can raise an Oscar with Guppies.