Richard

.. Nothing I've read on any of the sites I went to help me to tell the sex of a mature Oscar.. can anyone help me do that?

Chief_waterchanger

The best way to sex Oscars is by visual inspections of their sex organs. The females ovapositor (egg placer) is shaped like an ice-cream cone, while the male's sex organ (I can't remember scientific name right off hand) is shaped more like a pencil or pen.

A second though less acurate method is that females are generally thicker in the body area above the vent (the pooper), whereas the males are slimmer.

Gunnie

The chief might have something there, but everything else I've read about sexing oscars is that it is almost impossible to tell the males from the females until you actually see them spawn. I've had mine spawn once in my 100 gal. tanks, but didn't see them spawn (just saw the eggs), so I'm not even sure I don't have 2 females! The eggs all disappeared by the next morning.