I'm new to using OPKs for this cycle, but I've charted since 1/2010 (except when I was pregnant). FOR ME, I'd keep using OPKs if I didn't think FF was right. In fact, I've been doing that. FF had me on 8 dpos yesterday until I charted another temperature and CM, then it took away the cross hairs.
I was 99% sure FF was wrong, so I've kept up with the OPKs. (Still haven't gotten a positive yet, but that's a different story.)
My answer: if you think there's a decent chance of FF being wrong or you haven't gotten a + OPK yet, you can keep using them. In your case, FF looks to be pretty good, it has your O date on the day that you got a + OPK, so I'd lean towards trusting it.