Fletch, I don't think an OPK would do anything different just because of AF. I think it only picks up the LH surge around O time and some OPKs are sensitive to HCG because it's similar to the hormones present around O time.

Not entirely sure if the line has to be as dark or darker than the control, I have heard that though. On the other hand I've heard that some girls never get a 2nd line on an OPK until they are ovulating, so to see a 2nd line just before AF is a good sign for them. I always get a faint 2nd line on OPKs though so personally I'd want it to be nice and dark before believing that it was a sign of PG.

Good luck!!