Likelihood of getting pregnant without positive opk?

M
Is it slim to none that you can get pregnant without a +opk? (clear blue) tested once a day from CD12-18 abd then twice a day from 19-now I'm at CD25. My cycles have been 31-40 days. Am I likely not ovulating? Last month no +opk (40 days) but two months ago 31 days and +opk on CD15. What are my chances? Anyone with a bfp without a +opk?