You can find out how to top up your O2 prepaid credit here..
You can top up your prepaid credit for O2 tariffs in various ways. You can do this via app, online, with a credit slip, by bank transfer or with an automatic top-up. In this article, we will explain to you exactly how the respective procedures work.
If you do not want to permanently store your bank details with o2, you have the option of transferring money to an o2 account:
For an automatic top-up, you have to issue a direct debit mandate and enter your bank details . You can do this either directly when registering, or afterwards via the My o2 app or website.