I would even say that you could have 1 oscar in there (nothing else) if you took really good care of the tank (lots of water changes and filtration!!!).
over here in canada a 55 gallon is, 48 long x 12 wide x 15 tall. and no you cannon keep a oscar in a 55 gallon, i think the minimum is 100 gallons. and if you put convicts in the tank you might have a problem with aggressiveness, the convicts get very territorial when they breed and they breed like crazy, you probably couldn't keep anything else in there, enless you only got 1.
But I have kept oscar in the tank before. He died coz of electrical shock. I don't know how the wire break out of the power head. But funny, my pleco didn't die...amaze