The easiest way to sex an Oscar is as follows.
Get the Oscar's attention with teasing dropping of food at the surface of the tank. You want to get the O swimming as vertical as possible.
Then start taking some pics of its vent area.
Then in editing of the pic you can really zoom into the vent.
This is a female, you can see that one hole is clearly bigger than the other. In male's these holes are the same size.
Male fish
Tail oo head
Female fish
Tail Oo head
The forward hole is the anus, the rear hole is it's sex organ. The females eggs tube while inverted makes a bigger hole than the male's parts does.