ZERO. Need swimming space. They get 12" + easily. If you really really want one, get a 100+ gallon tank -- you will not regret it. I know most here would disagree, but OSCAR fish are incredible and rewarding fish to own -- if properly taken care of --- start off right by getting a larger tank.
I would say 1 or 0 depending on the dimensions of the tank. Bow front tanks tend to lack in length which isn't ideal for an Oscar. I wouldn't have 1 in anything under 4ft long.