You can use this form to cancel an o2 DSL contract. Please fill it out, sign it and send it to o2 by letter or fax as indicated on the form. Please note the agreed period of notice.

If you have received a DSL router from o2, it must be returned to o2 at the end of the contract. Please use the o2 DSL returns portal for this purpose.

Hi @britneybritney,
welcome to our community. It's a pity that you want to cancel your contract with us.
I am not sure if I understood your question correctly: Do you have a contract with us or not? Do you mean a monthly cancelable flex tariff? If that's what you mean, you can simply cancel this monthly in writing. A form is not necessary.
If you have a 24-month contract, the form linked by @blablup is the right one for your request.
Please keep us informed.
Best regards
Larissa

@o2_Larissa 

Warum sollte das Formular nicht für o2 DSL Flex-Verträge geeignet sein? Deine Antwort liest sich zudem so, als wäre für die Kündigung eines o2 DSL Laufzeitvertrags die Nutzung des Formular notwendig, was eine rechtlich nicht zulässige Einschränkung wäre. 

Hi @blablup,
can you say it in english, please? (My english teacher always said that)
I thought you linked the form for the foreign cancellation and therefore asked again exactly which minimum contract period the contract has.
Incidentally, you linked the form, not me  I also did not write that it must be used, even if you seem to interpret it that way.

Best regards
Larissa

I understood the TE to mean that she wants to cancel an o2 DSL Flex contract. That is why I have linked the normal cancellation form, as you can easily see by clicking on the link. Linking the form seemed appropriate to me, as the TE apparently does not know what to write in a termination letter. 
Incidentally, there is unfortunately still no form at all for cancelling an o2 DSL term contract due to moving abroad. This is still only available for o2 mobile phone contracts.