You most likely have to vent him or her. I'm looking for a mate for my oscar who I haven't vented yet.
If I vent it, I really have nothing to compare it with... so I might not be able to determine if it's a male or female... I can try and take a picture of his/her belly, don't know if that would help though... see article on venting below...
To Vent an Oscar you must first remove it from the tank and turn it over onto it's back so it is belly up. You must then examine the Vent.
The Vent is the opening between the anus and the anal fin. This is where the male fish release their sperm and the female fish release thier eggs. This area is also known as the genital papilla.
The female's Vent will be larger than the males to accomadate the passing of the eggs.
Unless you have one of the opposite sex for comparrison it will be difficult to determine sex for the unexperienced hobbyist. It will also prove to be more difficult with younger fish rather than more mature fish.
For sexually monomorphic fish species (females and males look alike) Venting is the most accurate form of sex determination.
Read more:
http://wiki.answers.com/Q/How_do_you_vent_Oscar_fish#ixzz1OEwmjApN