To a point, yes, any tank we put fish in is going to be much smaller than they would experience in the wild but the fish should still have enough room to grow to its potential, and a 55 isnt enough for an Oscar IMO. It's not just about "living". Put the same potentially 16" fish in a 125 and it'll easily grow large and live long. It's personal preference and plenty of folks keep Oscars in 55's. It can be done and "is" done. It's just not my recommendation. I prefer space.
Well said