Unfortunately, imo, Oscars are one of the hardest fish to get dithers or other fish to live successfully with.
Oscars grow so big and anything small enough to fit in their mouth will be eaten if they can catch them and within the confines of an aquarium that will usually happen. On the other hand, Oscars aren't as aggro as most fish their size and they will either get bullied or killed by some South American fishes and most Central American cichlids.
What size tank do you have and what filtration and other fish do you have so far?
Do you want another fish that grows to a bigger size also that can live cohabitate with your Oscar or are you looking for dither fish?
I'll be more sure of tank mates once I know filtration, tanks size, etc and what you are looking for, but I think GT's (Green Terrors), Jack Dempsey's and that size/aggro will work out best. Now there are some GT's and JD's that will be too much for an Oscar and I'm just generalizing here.
If you want dithers you could try Giant Danios or even better, Silver Dollars.
PLease give tank specs etc though.