Is this about prepaid or postpaid?

Moin,

for postpaid contract, no. 

If it is post paid and the top-up balance is under the same account number, than o2 did not use direct debit. 

If you have 200€ top-up than o2 uses your bank account after it is zero.

if you have an old contract switched and you mean Gesprächsguthaben than it didn´t work.

Hello @mugsie99,

welcome to our o2 community 

I read that Denner and schluej have already explained.

If more questions arise do not hesitate to ask here 

Best Regards,

Gerrit