A 55? Well, I'd say that's a good bare minimum (with LOTS of water changes). However, I will say I've seen a few Oscars in my time that would be unable to turn around in a 55 (all of them raised in large ponds, btw.) Raises the question of just how large Oscars would be getting if kept in larger systems (with better, more consistant water quality). I've heard some people say that get two feet in the wild...I don't believe that for a minute, but I do wonder....
I mean seriously how many oscars have any of us seen in our lives that were ALL THIER LIVES in a large system, and fed a good, varied diet?