Ditto what Shelagh said.
I've been growing my baby O's out in a bigger tank than that and they are almost ready to move out to the big one(I've had them 2/3 months or so.)
Oscars are usually easy to get hold of so I'd wait till you have the bigger tank then find one, there are usually people trying to rehome them on bigger ones for sale in LFS that people have returned.
Believe me, it's worth the wait as they are great fish to keep in the right conditions.