Saying "O" or "Oh" instead of Zero - when to use "O" or "Oh" ?

When can I use "O" or "Oh" instead of saying number zero ? Is using of "O" or "Oh" instead of zero correct in english or is it a slang ?

In english, there are multiple colloquially used terms for zero. For example, depending on country, you can come across with O, Oh, Nil, Null, Nought, Naught etc.

O or Oh is usually used instead of zero in a sequence of numerals, especially when spoken. For example when someone is saying a telephone number, IP address etc.