Oscars are monomorphic, meaning that male and female look the same. You cannot tell the sex of an Oscar by the shape of its fins, the length or shape of its body, or even it is colouring. You will normally only know the sex of your Oscars when they start laying eggs. Or there is always an off chance you can spot them with their genitals out during a teretorial display, dominance display ect. Females have a small blunt ended "egg tube" or ovaduct that protrudes posterior to the anus and anterior to the anal fin. Males have a small seminiforus duct or "penis" if you will, located in the same place as the female ovaduct. The male genitals are distinctly pointed in comparison with the blunt or rounded shape of the female genitals.