OPK's work by dectecting the surge of Lutenizing hormone that occurs 12 to 36 hours before O. YOu generally start testing a few days before anticipated ovulation. Rule of thumb is to test at 2pm and 8pm. At first you may get a few days of faint lines but these are considered negative. A true positve is when the second line goes as dark or darker than the test line. Once you get a positive O is likely to occur with in the above stated time.
OPK's don't work for everyone. Some people never get a true positive and stil manage to get a bfp. They can be expensive too. I have just used the lullaby conception brand and it was good. Others get them from ebay.

I used them this month and I was surprised to see how well my fertility signs matched the opk result. To be honest now that I have used them I won't go back because they really helped me pinpoint O.

For more info check out Fertily friend