You should be aware, that you may never be able to put another oscar (or any other cichlid) in a tank that size, when the current resident, and owner views the entire tank as its own private territory (unless adding a hot to trot female, and the current resident is a male).
It will see any other cichlid (oscar or not) as an interloper encroaching on its private space.
Most oscars defend a territory of about 250 gallons square or more, from interlopers in nature.
This is why is it almost always best to stock a tank with juvies all at once in the beginning , and let them work out the space together, never adding any similar looking fish again, (like other cichlids with cichlids). This could also go down hill quickly, with maturity if the tank is too small.
You might be able to add non-cichlids, because the perception by the cichlid of a territorial invader, is entirely different .
I realize this goes against the grain of many fish keepers, but it is often, the reality.
