Hi Min,

I have never had too much luck with OPK's (i always tend to get negative results even though I know through temping and CM that I have O'd). However I believe that the difference between sperm and cm is that the fertile cm (ie EWCM) is more clear and stretchy. It should usually stretch between 1 and 2 cm's and not break. Sperm I think is more whitish and and is more likely to break when pulled. I hope that this helps, perhaps there are others out there who could shed some more light.

Good luck.