Positive day of O or just before?

So for a few months now I’ve had negative opk strips on the actual day I’m ovulating, at least I’m pretty sure it’s the day because that’s when I get middle cycle pain so bad that I can hardly stand for a few hours. Usually on day 12 but sometimes 14 (28 day cycle). Sooo are opk strips generally positive on the days leading up to O and then it drops the day of ovulation??? Thank you.

Ps these are assured dollar store tests. Not sure if that matters. Day 12