I've used OPKs. From memory, the way they work is by detecting your LH surge (luteal hormones). Aparently you have a surge of these hormones between 12 and 48hrs before you O. I would test from about cd5 with the OPKs, until I got a strong positive, and would keep testing until it turned neg again. Usually I'd get 2-3 positives in a row. As soon as I saw the first + we'd start DTD, and keep it up for a couple of days after the OPKs were - again.

Sure enough I got pg our 2nd month trying, so for me, they worked really well.

If you felt your O pain today though, it's probably too late to detect the surge, hence the - test. But I'd still DTD anyway for good measure. An ovum will survive for 12-24hrs after being released. Hope that was helpful! Good luck