It's not wise to mix true tropical fish with goldfish or koi. Oscars are in the aggressive category and should only be kept with larger, equally aggressive species. Your pet store dealer can help you select compatible species. Make a list of which fish you like, go over it with your pet store dealer and then together come up with a combination that works. You must be careful not to overstock your aquarium.
For more information, please read How to Select Ornamental Fish for Your Aquarium