The only way to know for sure is to temp. I know some women who O the day of a +OPK and some who, like me, O 2 days after. You can track CM and after O it will turn creamy but it may not be the day after. I guess another option is to count how many days from your +Opk you get AF and at least you will know about how long you need to wait for AF as they time between ovulation and AF should not change by more than one day.