I have an oscar, a jack dempsey, two plecos, and two convict. i bought them all at the same time and they were always best buddies. they never fight and are always swimming around together.
when my oscar outgrew the tank it was in, and my friend had to hold him for me. he started mopeing around . then my friend bought him a companion (a Jack Dempsey) he ate it .still saddened, we bought another oscar for company, he came right back to life . when i got him a new tank and put them in with his old buddies he was happier than ever.

Moral = Oscars are happiest with others, but be careful not to put somthing in that he doesnt like because what an oscar doesnt like and oscar will eat.