hi Tess,
Like you I have had irregular periods since stopping BCP. I wouldn't even try OPK's if I were you as it's been 6 months since you stopped pill and you are still having very long cycles. So, my advice would be to check in with your GP about your long cycles, which indicate that you may not be ovulating. You might need to see a specialist to work out if you are ovulating (this can be done with a blood test). If you are in no hurry with TTC then you could wait a bit longer to seek medical advice.
Unless you test with OPK's every day from about day 14, you may not catch the surge. You might end up using heaps of OPK's this way but it would be the only way to detect if/when you are ovulating.