I have a 28-30 day cycle and started using the OPKs around CD12 through to a positive or to CD18. If your cycle is about 33-36 days you count back two weeks from the end of your cycle so around CD 19-22 would be about when you should O. So in that case maybe start testing CD16 or so.

I also used some I bought off ebay and they worked just fine - although often looks as though the line is not darker than the control line, but I found that to be a positive anyway. Just something to keep in mind.