Ø is only used in
Danish and
Norwegian to my best knowledge. The
Swedes liked the
German letters more, and therefore they have ä and ö instead of æ and ø (æ not to be mistaken for the
French œ which is actually
pronounced just like the Danish/Norwegian ø (and
Swedish/
German ö).
As you sometimes find yourself in a situation where you don't trust the special characters on your keyboard, you can use oe instead, which most people in all five countries understand (Oe in uppercase).
Another
nice useless fact is that the word ø actually means
island in Danish. And the danes actually have a meaning for the word
å as well - which they share with the Norwegians
AFAIR)