If you only get one positive OPK per cycle...

KMLP

on which day do you usually O?  Typically, I get two positives and I ovulate on the day of the second positive or the day after the second positive.  This month I only got one positive.  I'm taking a break from temping this month.  

jiffy-pop

I'm lucky to get a true +OPK, even when I do O. I sometimes have lines but they are not dark enought to be considered a +.

ETA: The only +OPK I remember getting, I think I o'd the next day or maybe a couple of days after.

btaylor19

I usually O 2 days after my 1 positive OPK.

ALF222

I usually O the day after my +OPK with FMU.

MBfromBMC

OPKs never worked that well for me, but the RE said that you should O 36 hours after the +.