Lösung von o2_Jessica
for the DSL contract you do not need a special form. You can just write a cancellation, with saying you're moving abroad, and also hand in the deregistration paper of Germany, the dergistration paper of your new home or a confirmation of your new employer as well as a letter of agreement that o2 is allowed to charge 3 monthly basic fees.
Keep in mind that you also have to send back the router from o2.
The address for the cancellation letter is this:
Telefonica Germany GmbH & Co. OHG
Postfach 60 09 40
For the mobile contract you need a special form and that you find attached to my answer here.
All further information you find in that, also the address and the needed documents.