I already O'd this cycle, but just wanted to let you know that this was my first time using OPK and I got my positive test on the day I O'd (according to my fertility friend chart). And if I hadn't tested twice that day I would have missed it all together.
I tested @ 3pm every day starting with CD10. I never saw anything other than the control line until CD16 when I saw a VERY FAINT test line (obviously not + since it has to be as dark or darker than the control). I tested at 8:30pm that same night and got a clear +. When I tested the following day @ 3pm (just to see what I'd get) I got a VERY FAINT test line.
So it looks like I faded in and out within 24 hours and had I not tested twice on CD16 I would have missed it. Good luck, I hope you get your +OPK soon!