Question regarding ovulation

Madison

When y’all talk about “DPO”, what exact day are y’all talking about?

For example if you’re using an OPK do you use the day that both lines are positive, or do you technically use 1-2 days from then because that’s when the OPK is predicting ovulation. Or do you use the day when your app says you’re going to ovulate?

Trying to find my cycle buddies that I can follow along with! Thank you!