the opk will show a positive 24-36 hours before O. that is because it is not actually measuring O but the surge of LH in your body. and you get the LH surge a couple days before you ovulate. you can also have an lh surge but not o.that's why i said before that my doctor told me that opks are an "indirect " way of measuring O.
however, if you had EWCM as well as positive opk you should be in good shape.
as far as starting the bd a few days before you ovulate, that is because sperm can live for up to a week in your body,
in fact my sil got pregnant even though she and her dh did the bd more than a week before she ovulated. in fact they were not even ttc. they had a 6 month old and she was breastfeeding. so she thought that would act as a natural birth control. wrong!!
anyway, sounds like you have all your bases covered with the cm and positive opk. so the best of luck to you and i'll keep my fingers crossed for you! lots of baby dust coming your way