How to count dpo.

Samie • Samie, 22, University Student, Fiancée to Adam, mummy to Lilly Jayne & expecting baba number two in August :)
When do you count dpo from?
​First fertile day, day after positive opk? When?
​I usually feel sharp pain on one side when I ov but I got a positive opk on Monday & haven't felt anything yet!