I haven't made a post in over 200 days due to no interest in what people post here but I do love aquarium tanks. Having a 55 gallon upright fresh water tank myself. I figured I'd add my input.
I don't have extremely aggressive fish in my tank but I have had them before. The problem with Oscar fish is they're very territory dominate. They typically do not like fish any smaller than themselves. So if you can purchase a cichilds or discis that are at least 3inches long. They should live peacefully together. You know what's a good tank mate for them is a random pond fish. They're very aggressive and will keep the Oscars in line and on there feet. Only problem with pond fish. It's sometimes hard to get them to adjusted to being hand fed.