a 55 is 48inches long and only 13 inches deep. Eventually the oscar will have a hard time turning around.
theres lots of ways to get to 55 gallons, check out tenecore's site for a good refernce on a few differnt 55 configs. In fact, if this guy every gets back to me, I'll have a 36x18x20 very soon, check the volume on it. its perfect if you want some extra depth, but unfortuantly, they aren't what most stores carry.