You can assume, but allow that voice of doubt to still be in the back of your mind because you can gear up to O, your body decides "eh not yet" and then shuts 'er down only to resume the process in a week or so.

I think it's best to look at the bigger picture, things like libido, PMS, EWCM, O Pain, post-O pain, sore BBS, OPK if you're using them, etc I have a pretty good idea when I Oed but not even I can get it to the day. I'm pretty good at it at this point but I could not tell you if it was yesterday or today or tomorrow that the egg actually popped.