I don't know this is always true or not, but according to my experience, male oscars breeding tube usually can be seen almost all the time when they are mature. especially if other oscars are in sight. its pointed almost backward and might be very small and hard to see ( in non breeding situation ) but theres something there!
but females breeding tube can not be seen in normal situation.
thats my own experience, correct me if i'm wrong please.
There is no proven way to sex oscars. There's a lot of theories, but nothing solid besides watching them spawn. some will say the "eye spot" on males travels up the dorsal fin. But both my females have that. During courting some will say males are more aggressive, but I've seen it the other way around. Some will say the female will do most of the cleaning on the flat rock provided for a spawn, but my current male does all the cleaning :lol:
As for Blu-rays theory, can't say I've ever heard that one, or noticed it in mine :-?