Nature is much larger than a 55g tank. If your tank was much larger, a school of angels would likely work. Oscars in nature school, problem is, most people don't have tanks large enough to house a school of oscars. There is also a big difference between a school and a pair. A school allows for the dispersion of aggression among many individuals. With a pair, one fish is taking the entire brunt of the beating. Cichlids overall do not tend to play nicely with each other.