How do OPK’s work?

Hannah

15mths TTC. Considering getting OPK’s this month. All we’ve been doing to ‘try’ is taking my BBT and trying to have sex every second day over the fertile window. It’s getting frustrating and wanted to try something else but don’t understand what OPK’s will do? And when pos to be is that a good time to have sex or should you do it before positive?