Have you been taking your temp? As kirsty says, OPK's are great to give you warning you will O in the next 12-36 hours, but by also taking your temp it will pin it down to when it has happened due to the temp rise.

Problem with using only CM for working it out is that you can have ewcm at times in your cycle when you are not about to O, so abit unreliable.

Carys