piranha45 said:oscars (quickly) grow too large for 55g tanks. A 75g can hold 1 grown oscar, though I would recommend at least a 125g.
your only solution is to remove the bully, take him back to the LFS or euthanize him. There is a high chance that a new bully will arise from the remaining oscars, though. So you may end up removing a lot of fish. Oscars are big fish that need their space.