It's also worth mentioning, if you have irregular cycles get one that has more than 2-5 tests in it. Those should only be used in a female who has very regular cycles.
I just used the Wal Mart Brand to get pregnant and got pregnant on the first try, but my husband had just came home from Iraq so we spent a lot of time being intimate. Just don't stress and use whichever OPK you feel might be better for you and enjoy being with your partner!
I used clear blue the digital ones and got pregnant the first time. Good luck
OPKs work by detecting the LH surge that occurs before you ovulate. Usually you will ovulate 12-48 hours (rough estimates, I'm not 100% sure on them) after you have a positive test. On the flip side however, it's possible to have a surge detected and then you fail to ovulate for whatever reason.
They tell you when you are about to ovulate...i have used first response both times with no luck but thats just me...my friend used clearblue and got pregnant first try...i havent tried anymore but maybe i will if i dont become pregnant soon