Fish do not grow to the size of the tank. They may stunted and sick, but will grow.
For one Oscar, you need at least 50 gallons.
If you want fish, I would suggest maybe 5 - 7 Neon Tetras (or other small tetras(, 3 Harlequin Rasboras (Or two female and one male Cherry Barb) and maybe 3 corydoras. That is plenty for a 20 gal tank.
The biggest mistake new fishkeepers make is to overcrowd tanks. I learned this the hard way many years ago.