When using opks I test about 3 times a day coming up to O because if you do it only once a day at the same time you could only be getting the end of your surge. Then you wonder why it is negative later or the next day. For example, the day I got my +opk I got it about 10pm at night but I did also test twice earlier and it was negative. But the 2nd line was getting darker and I knew that I was about to get my surge. So I got it later that night and the next day, if I tested later that next night it may have been negative (but I don't know since I ran out of opks). So yeah, I suggest testing noon, dinner time and night time when coming up to O and also using ICs since it would be expensive doing so many.