OPKs or ovulation predictor kits are tests that can be used to help you determine if and when you are ovulating. They work by detecting the level of gluteinizing hormone (LH) in your urine. In general, the level of LH rises just prior to ovulation. Using these kits during the range of possible ovulation times during the month allows you to pinpoint that surge and gives you an accurate estimate of when ovulation will occur.

For women who may be suffering from fertility issues, these kits help determine if ovulation is occurring at all. This can be confusing, but you can still have periods even if you don't ovulate, so even a regular cycle can be marked by an absence of ovulation.

Most women will ovulate within 14-34 hours after the OPK FIRST becomes positive.

You should start testing with the OPK about 4-5 days before expected ovulation.
Here is the first day you should start testing based on your average cycle length:

 

Average Cycle Days	First Cycle Day to Start OPK Testing
21	3
22	4
23	5
24	6
25	7
26	8
27	9
28	10
29	11
30	12
31	13
32	14
33	15
34	16
35	17
36	18
37	19
38	20
39	21
40	22