Counting DPO??

Tina
I think I posted this question already but I'm not sure. If I got a positive OPK on the evening of May 18th, does that mean I count my ovulation day on the 19th or 20th? Or do I count my ovulation day the 18th since that's when I had the positiv OPK? Just trying to figure out when to count DPO. Thanks.