Typing
To type the Danish "o-slash," like in Nørrebro, on a Mac, type Option + o.
For Windows, it says here, you need to hold down Alt and, on your numeric keypad, type 0248.
The HTML entity is ø.
Talking
"similar to German ö sounds like e in let pronounced with lips tightly rounded" (learn DK headlines)
"To non-rhotic English speakers, the vowel it sounds most like is the vowel in "bird" or "hurt"" (Wikipedia)
SpeakDanish.dk (with audio)
"To pronounce the vowel, begin with an ay sound as in bay. Because this is actually a diphthong in English, you need to cut off the final ee sound, leaving a pure vowel. It should strike you as foreign. Once you have this sound, you merely need to round your lips as if you were saying, "Oh!"" (everything2.com)
It's interesting —
— that the three methods produce surprisingly similar sounds. Isn't the Latin alphabet great.
The question is, how do you pronounce the trailing 'o' in Nørrebro?