Oscars start off growing very fast, and like most fish the pace slows down as they get bigger.
75 gallon is more recommend because these fish can grow as large as 12", or larger. The tank width of a 55 gallon is 12". You can see how space is an issue. The extra 6" of depth on the 75 makes a difference.
With that in mind, it will be a long time before it really grows that big, so a 55 would be fine for a while to come.