Absolute minimum is a 55gal for a adult Oscar, prefer 75
gal or larger.
And you'll be needing it sooner than you think, O's grow at a average 1" per month their first year and are
very very messy eaters/poopers, you'll need to have good filtration, atleast 10x turnover with
HOB and 5x with a canister filter.
By the time you start thinking he's getting too big, he will be already due to the high bioload.
If you want to keep the Oscar ( they are great fish, very friendly) you'll have to get a bigger tank, period. Any thing less than a 75gal and the Oscar needs to be by his self
IMHO.
You can keep other smaller Cichlids in the 30
gal, Bolivian Rams, Kribensis, Jade Eyed and etc...