Hi!

You have the following options in Mein o2: https://www.o2online.de/ecare-prepaid/automatische-aufladung

You can set an otomatik top up in Mein o2 to charge a certain amount on a given day.

Remember that almost all options have a maturity of 28 days!

Therefore you should better use the following option. 

Charge the credit if the credit for activation or auto-renewal is insufficient

Another option would be to set up a standing order (Dauerauftrag). 

You can set it up at your bank. The necessary information you get when you send a (blank) SMS with your o2 prepaid card to the 56656. As an answer, you will receive the bank details and your personal intended use (Verwendungszweck) by SMS.