elly, do you know when you O?
It would be a waste if you start using OPK's so soon after AF if you don't O untill CD14 as you would have so many -ve ones. If you get any EWCM then use one but maybe wait untill CD12. I've had cycles where I didn't O till CD18 and wasted a whole packet of OPK's. They are very expensive, especially if you are using them every cycle.
I've used Fortel and it has a guide that gives a good time to start depending on your length of cycle.