Do not put an oscar in a 55. First they will get so big that they can not turn around in the tank. Also they can not swim backwards. SO.... no to the 55... maybe africans would work.
Oscars do get quite large I would think that they would be hard pressed to have other tank mates. I think you might want to go with a few smaller ones.
i had a convict in my 55 gallon for a while. they were both happy. but when i put another one in there it was just too much. now i have a happy 75 gallon and my 55 is my oscar's tank.