OPK stands for Ovulation Predictor Kit, you can purchase them at your local drug/grocery store it is located in the same area as home pregnancy tests, and you use it the same way by peeing on the stick, if you get 2 dark lines then it means you should ovulate in the next 24 to 48hrs, I usually get the Walgreens brand that has 7 tests, and it is cheaper. I always start using them on cd 9 or 10 until I get a positive which is usually on cd 14 & 15.
The bad thing I hate about OPK is that the best time to test everyday is in the afternoon hours I usually test around 3 or 4pm, you have to hold your urine all day and don't drink anything because fluid will dilute the hormone and your test will be negative.
Check this site out and it will give you more info about it.
http://www.babyhopes.com/ovulation-predictor-kit.html
Hope the tests work for you and you get a

I have used them many times but no

for me yet. I have read sites were it has worked for lots of women TTC.