You could probily put one or two more medium size fish in with the Oscar , but dont forget that Oscars can up to 12inch them selfs so you shouldnt really get anything that will not grow to about the same size , so the Oscar will not eat their tank mate (s) or that the Oscar wont bully the other fish .
Jack Dempsy , Severum , Acara , Pleco , Jaguar , Convict , Fire Mouth , Texas .
Maybe a Lima Catfish or a simaler size catfish which wont eat the Oscar them selfs .
There's just few diffrent Fish that i can think of at the moment .