You could go with other cichlids of similar size or large catfish. It really depends what you want. How big is the Oscar ? What are the tanks diamentions?
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Quote a bit can go in with an Oscar; other SA and CA cichlids of similar temperments for a start. Of course, just like when mixing all cichlids, make sure to have a back up plan, just incase they don't get along. A spare tank helps.
Avoid fish named after prized fighters, devils as well as terror inducing names and you should be ok.
Oscars are fairly tough but by no means aggressive fish. Although, anything that will fit in their mouth should be avoided, as they are pigs and not picky eaters.
Start by looking around and see what you like. By the time you figure out what you want for sure you should know whether or not it will work with your O.
Good luck with the stocking and let us know how it works out and post some pics!