The vent pic to me is only thumbnail size, and I don't know how to view it otherwise, but oscars are not sexually dimorphic; they do not show outward physical sexual differences. I would trust a professional breeder to vent-sex an oscar, but I put faith in the old "the one with the eggs tumbling out of it is a female" theory.
I had a pair of oscars that I assumed to be male/female. Early on, I had my red pegged as male and she surprised me by laying thousands upon thousands of eggs. Pretty soon after, her mate, who had completely different physical characteristics laid her own batch of eggs.
Even if you take into account that you really can't tell, you can't help but assume you know who's got what. And then they keep surprising you.
