Well with + OPK it usually indicates that you will O in around 48 hours (give or take) - so you would have BD on the day of ovulation, you may have caught the egg on it's way down. It's best to BD around 1-2 days before ovulation as it takes the sperm a little while to reach the egg. But thats not to say you're not successful.
Good luck. Remember for next cycle if you don't get a BFP to BD a couple of days leading up to O - the more BD sessions you can get in right before O the better!
