You can tell if your oscar fish is male or female by assessing its reproductive area after they turn one year old. Male and female oscar fish have two holes in their reproductive area, the anus and the sexual organ. In male oscar fish, the sexual organ resembles a small spike. Meanwhile, in female oscars, the organ appears blunt and slightly wider.
With that said, determining the sex of an oscar fish by analyzing its sexual organs is extremely hard. Even experts occasionally have problems with this.
There are no other ways of sexing oscar fish. Oscar fish are monomorphic, which means the males and females look exactly the same. Thus, it’s impossible to tell the sex of an oscar fish by just looking at its color, shape, or size.