last time I read a thresd like this I wasn't very nice as I assumed it was someone trying to get everyone going.. it wasn't
so trying it the right way this time..
If you already own the aro you either need to rehome it or put it into a much larger tank asap, the idea they grow to the size of the tank is not true, being in a to small tank does lots of harm to the fish even if you can't see it. For an idea of size of tank need and the size these guys get look at some of the pictures posted on this website. Oscars are very dirty fish and will out grow your tank in a month or so.
If you want to stay with the small tank there are alot of nice looking fish that could be happy in it for life. If you want to go with the bigger tank, great, get it right away and then read the stickies on here about cycling tanks.
Most people here if they understand you want to do the best for your fish are more then willling to make sussgestions, and offer help if needed.
Gl and hope you do the right thing for your fish