Can you? Yes. Should you? Probably not. Oscars commonly reach 12 inches and occassionally 16, so most folks on this forum would recommend a 75-gallon tank as the minimum. You can also keep a Rottweiler in a 3' X 2' crate — but you shouldn't, for the same reason.
I think it would be OK depending on the width of the tank. A 55 tall isnt wide enough for a full grown oscar to turn comfortably but if he has a 48 wide it may be possible. And it would be more like keeping a rottweiler in a bathroom or walk in closet, which is better than a crate, but still not big enough. Not trying to start an argument, just trying to be more accurate cause the 3x2 crate mental image was disturbing.
yeah that was kinda what I was thinking too. if the tank is wide enough for him to turn around comfortably it would probably be ok, but if it's a regular width for tanks that size (what is it, like 12 inches I think?) it might be a little too tight.
the 75s are commonly suggested because they are typically the same length and height as a 55, but they are a fair amount wider, which gives the larger fish plenty of room to turn comfortably.
I gotta agree w/ 'monkey_toes' on this one. (been there & it ain't no fun)! The 75gl would give the adult 'O' the space & volume to "THRIVE" rather than just "LIVE". A 48gl, although sufficient for a young 'O', would be outgrown in less than a year.