It is very difficult to sex O's. Venting is the only way you will know for sure until they are nearly a foot long. Once that big IF you can get someone to distract them at the top of the tank, check the tubes. Males will have 2 same size circles. Females will have one small tube and one large tube. It is very difficult to get them to cooperate while you check.
Male = oo
Female = oO
PM me if you have questions on O breeding. Have had 2 breeding pairs and feel like I halfway know what I'm doing