OPK, Ovulation Predictor Kit
Here is my personal experience with using them.
I find that some brands are more sensitive then others. Some Dollar Tree's carry OPK's. I will get Answer OPK's or I will order them from www.SaveOnTests.com.
I start testing on the first day I start getting fertile cervical mucus. I test at least twice a day. Once in the afternoon and once in the evening. Once I get a positive I will usually ovulate 12-48 hours later. I have gotten what I thought to be a positive and then it's gone back down for a day or two and then had it go positive again. This is why I love charting. If I didn't chart I wouldn't know for sure when I ovulated.
Here are some pictures of some of my OPK's. Click on photo to enlarge.
Dollar Tree Tests SaveOnTests.com
A word of caution, PLEASE DO NOT use OPK's as HPT's. I tried this one month and would get positives in the morning and negatives in the afternoon/evening.
All it does is leave you with more questions. If you think your pregnant then take a HPT.
OPK's are only one of the tools that you can use to help you try to determine when you MIGHT ovulate but it is not the finale answer to that "Am I going to/Did I Ovulate" question.
Click to enlarge OPT/OPK Timing Chart.