Oscars are a very popular fish to own as pets; they come in many different varieties which offers fish hobbyists a large selection of colors, fin styles and sizes. Determining the sex of an Oscar can be important if you intend to breed them. They are known for pairing with each other and remaining faithful, so many people choose to buy them in pairs. Oscars are monomorphic, which means that the males and females are identical in appearance, requiring special inspection to identify their sex.
Wait until the Oscar is sexually mature. Depending on the tank size and the Oscar's health, the fish can mature at different ages. Healthy Oscar fish in a large aquarium often become sexually mature at 1 year old and once they reach 6 to 8 inches in length.
Place a dark, flat slate or rock on the bottom of the tank. If there is no slate or rock for them to lay their eggs on, they will select a spot on the bottom of the tank, then push aside all the sand and gravel and lay their eggs on the flat bottom of the tank. A dark rock will make it easier to see the eggs and provide a surface on which the Oscar will lay them.
Inspect the tank daily and watch the Oscar's behavior. Before a female Oscar becomes sexually mature, it will often practice cleaning an area of the tank to lay her eggs. If you have placed a flat rock in the tank, this is likely the area the female will clean. If you witness this behavior, it becomes clear that you own a female Oscar.
Check the slate or rock you inserted into the tank often to see if the Oscar has laid eggs. If you own two or more Oscars in one tank, you will know that the Oscar which spent time cleaning a spot for the eggs is the female.
Check to see if there are any males in the tank. If you are trying to sex only one Oscar, it will become evident they are a male once they are older than two years and have never laid eggs. If you have multiple Oscars in one tank and have identified the females by witnessing them laying eggs, it becomes easier to identify the males.
Inspect the bottom of the Oscar that you suspect to be a male. When they are ready to mate, you will see a small protrusion from the bottom of them -- behind their rear fin and next to their anus -- which looks like a small thorn. This indicates that the Oscar is a male. If there is a small hole, it is a female.