i would say 5x2x2 for an adult oscar and a few friends. they do grow very fast so if a juvi is put in a 3ft tank it would soon outgrow it.
my big tank is 6.5x2x2 and has a fully grown oscar, 4 other 6 inch plus cichlids a pleco, 3 x catfish and a bgk and i wouldnt want to add anything else tbh. they have just enough room and if you have ever seen a 12 inch plus oscar you will know what i mean