I have a pleco, red devil, a pair of convicts, and a red tailed black shark with my oscar and they get along pretty well. Oscar and my red devil(felix) are constantly swimming together messing with my pleco. The convicts pretty much stay away from everyone else, and surprisingly the shark pretty much does whatever he wants.
This tank is a bit small, but check my gallery. Some suggestions there.
Silver dollars, CA/ SA cichlids, big cats, hoplo cats, plecos, a swamp eel... But it depends basically on the oscar's personality.
If it was me, I'd buy the bigger tank you're planning on before adding any tankmates. From personal experience I know how you always plan on upgrading but sometimes never do. LOL