What is the unambiguous way to differentiate 0 (zero) from O or Ø?

In languages, which don't have the Ø, essentially use it to differentiate between zero and big o. What does one use in Norway, given that Ø is already in use?

EDIT: I get it, that O is circular and 0 oval. Yet, when it comes to handwriting or certain fonts, I happily use (the oval version of) Ø, especially for passwords.