I think ff is having some strange issues this month, mine has dotted red o/cover line on cd 16 but I'm sure I o'd on 13. My temps are stupid because I have an over active thyroid but I get nearly a 1degree drop for o, it's telling me I o'd on a day I had a 0.15 degree drop.
But remember that opks detect the lh surge meaning you're getting ready for o, not that you are o'ing. And a few days difference each month can be normal.
I personally find opks too confusing and way too much variation between women so I just do temps, cm and cp but wasn't doing cm or cp this month, but I know lots of people who thanks to opks got a BFP, I'd say as long as you dtd a few times quite close to either, then you'll be in for a good chance ;-)