Hi kirsty,
I hope this is your month! OPK's detect hcg as well as LH and it has been my experience that they detect it before hpts. However, this is dependent on your body.
A great deal of women will only get a prominent opk line around ovulation and if you are one of those and you have a prominent line now then it is likely that your opk is detecting hcg. As was Sezjm's experience the line doesn't have to be darker than the control - in the early stages an opk will show a second line that gradually over a few days gets darker. However, remember that only a pregnancy test can give you a definitive yes or no. Did you test this morning?
I hope this is your month!