I have never had a true positive opk. I find them so unreliable. I was doing four tests a day around suspected o date, so as not to miss it. Held my pee, all the tricks they say to do. On Monday they were close to positive, but not quite. After they dried however they all looked positive with one edge particularly darker than the rest of the line. I noted them all as positive. A couple of days later my temps went up, confirming it.
I also have a two year old who was conceived when I took a "dark but not dark enough to be a true +" opk as a positive.
But if you're anything like me opks are very frustrating and unreliable. I just use them as a guide....