First of all it is not recommended to feed them feeder fish. Whenever feeding fish to another fish make sure that fish is the common in its natural habitat. When feeding feeder fish there is a good chance that fish carries some kind of disease. If you see the tanks many of those are overcrowded and you see fish dead or dying. The only fish that you can feed to your oscars are that of Bunocephalus, Rineloricaria, and Ochmacanthus genus. These fish are catfish and are naturally eaten by Oscars.
One way to tell is too look underneath the fish. There are normally 2 holes for male and female. For males the holes should be the same size while for females one of the hole is bigger!
Another thing I hope that you aren't keeping the Oscar in a 55 gallon since that is too small. An Oscar usually get 12-14 inches. That being said a 55 gallon is only 12 inches wide which means it will have a hard time turning when it fully grows.
Venting works on Mouthbrooding Cichlids, where the females have to lay big eggs. However, it is not usually effective with substrate spawning species.
IME, It is not possible to sex Oscars, even as adults, until they are ready to spawn. A past member of the forum who bred multiple pairs of Oscars regularly said he could not sex them unless breeding tubes were down.
As for food, if it crunches, it's good for them. Small shrimp, snails, crayfish, insects, and an occasional piece of fruit makes up the best diet. They are not piscivores, and while they will take fish if the opportunity arises, it is not a part of their regular diet in the wild.