When is 1 DPO?

It's kind of hard to judge tbh. Ovulation can happen anywhere from 24 to 72 hours after a peak ovulation test. As a general rule I think most people count from the day after, but I'm sure there are also many who count the day of.

Days past ovulation so the day you think you ovulate is Day 0. That means the next day, one day last ovulation is DPO1 etc

But as Melissa says the day you got a positive ovulation test is not DPO 0. It takes 24 to 72 hours after that positive test for ovulation to happen. I usually had a super minor cramp on one side only sometime after a positive ovulation test which would be released of the egg from the ovary on that side and actual ovulation so I counted that day (minor cramp day) as day 0

Ok so say I had a positive test Saturday evening, I'd assume I'm ovulating on Sunday, so Monday would be me dpo 1. But I think most people do count the day after the test.

Well if it was Saturday evening I'd probably assume I ovulate Monday morning so Monday would be DPO 0 and Tuesday would be DPO 1. But even though I knew how to get DPO I never really used it. I just tested first day of missed period for pregnancy :)