After O, I would have faint lines on OPKs until AF came. Some women have LH in their bloodstream all month long. It takes a while for it to taper all the way off after a surge.
HCG will ALWAYS show positive on an HPT before on an OPK. When I got pregnant, I used the rest of my OPKs just for fun and they didn't become dark lines until WEEKS after my positive HPT.