Naomi, the OPK should only show a second line around O time,,,,,although women with PCOS can show lines all the time (it's their screwy hormones). On the other hand, some girls dont get a second line, even when they are O, or they only get a faint one.
As for using the OPK as a pregnancy test, there are heaps of examples on FF, where people have scanned it for you to soo. It has been done before, but you would get far better results using a HPT I would expect.
I know I probably haven't been THAT much of a help, but it seems that every woman gets varied results and sucess using the opk's. Good luck hun, I hope you get great results from yours :d