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Sexing oscars isn't easy unless you definitely have a couple of similar aged mature oscars for comparison. Since you have a large number of oscars (and also hoping you have a equally large tank for them), you can try this. The male's dorsal fin is longer and often overlaps the tail fin. Also, the male's anal fin is much more pointed. A females dorsal fin is usually rounder. The male may also have 2-3 dark spots at the base of the dorsal fin.