never kept oscars but my trusty book says (paraphrasing):
+ grow to 38cm long
+ best kept singly or in pairs in a single species tank
+ absolute minimum tank size 100x40cm for a single fish, 120x40cm for a pair
+ Totally unsexable so a group of fry should be grown on to obtain a pair
+ Everything but the gravel rendered unmovable
The ultimate aquarium, ©1995 Mary Bailey and Gina Standford, Pg 134
The book has never steered me wrong before, but given the eventual length of these fish a would say that the minimum tank size is very very conservative.