Oscars can have many different kinds of tankmates if introduced correctly. You got to remember oscars tend to be aggressive, and will defend their terriory. Never put any fish with an oscar that will fit in its mouth for the simple fact it will make the fish a snack and not think twice about it.

Possible Tankmates Include:

Firemouth Cichlid: they grow up to 6 inches and usually if raised together will make an oscar a good tankmate.

Convict Cichlid: Highly aggressive cichlid that grows around 5 inches, but if introduced while the oscar is small it should be ok.

Other Oscars: Sometimes won't work well, but if you put 2 or more oscars together in a large tank while they are small they will be just fine.