Oscars are aggressive freshwater fish that are originally from Peru, Colombia and Brazil. Sometimes it can be very difficult to tell which sex these fish are because the males and females of this species have no color difference like with many other species of fish. The easiest way to tell whether your fish are male or female is by simply watching the Oscars. It is much easier to discover the gender of your fish if you have more than one Oscar in the same tank.
Observe your Oscars very closely. Watch for one of them to begin laying eggs, which is an indication that the fish is female.
Observe the underside of the Oscars in the tank. Male Oscars have a small spike or thorn-like protrusion from the underbelly. The small spike or thorny protrusion is his sex organ.
Watch to see if there is any mating activity going on. If you never see any sex organs from any of your fish, it is safe to assume they are all male. If no other Oscar is interested in an egg-laying female, then it is safe to assume that all of the fish are females.
Keep the tank clean and perform regular water changes. This will encourage the Oscars to breed, which will allow you to see which fish is which sex. If you do happen to observe a female laying eggs, watch for mating activity the next month, a few days ahead of when you saw the female laying her eggs. Oscars spawn every month.