The name of this letter is the same as the sound it represents (see usage). Though not its native name, among English-speaking typographers the symbol may be called a "slashed o" or "o with stroke". Although these names suggest it is a ligature or a diacritical variant of the letter o, it is considered a separate letter in Norwegian and Danish, and it is alphabetized after "z"—thus z, æ, ø, and å.
In other languages that do not have the letter as part of the regular alphabet, or in limited character sets such as ASCII, ø is frequently replaced with the two-letter combination "oe".
In Danish, ø is also a word, meaning "island". The corresponding word is spelled ö in Swedish and øy in Norwegian.
Ǿ (Ø with an acute accent) may (very seldomly) be used in Danish to distinguish its usage from a similar word with Ø. Example: hunden gǿr, "the dog barks" against hunden gør (det), "the dog does (it)". This distinguishment is not mandatory and the first example can be written gǿr or gør, the first variant (with ǿ) would only be used to avoid confusion. The second example cannot be spelled gǿr. In Danish, hunden gør, "the dog barks", may sometimes be replaced by the non-authorised spelling hunden gøer, this is however usually based on a misunderstanding of the grammatic rules of conjugation of verbs ending in the letters ø and å. These idiosyncratic spellings are not officially sanctioned. On Danish keyboards and typewriters, the acute accent may be typed above any vowel, by pressing the acute key before pressing the letter, but Ǿ is not implemented in the Microsoft Windows keyboard layout for Danish.
Ø is used in Old Icelandic texts, when written with the standardized orthography, denoting, among other things the umlautso > ø and ǫ > ø.
The letter Ø-with-umlaut (Ø̈, ø̈) was used by the Øresund bridge company, as part of their logotype, to symbolize its union between Sweden and Denmark. Since Ø-with-umlaut did not exist in computer fonts, it was not used in text. The logotype now uses the spelling Øresundsbron, with Øresunds- being Danish and -bron being Swedish. The letter Ø-with-umlaut sometimes appears on packaging meant for the Scandinavian market. For example, liquorice brand Snøre/Snöre's logo on the packaging is Snø̈re.
The letter "Ø" is sometimes used in mathematics as a replacement for the symbol "∅" (Unicode character U+2205), referring to the empty set as established by Bourbaki, and sometimes in linguistics as a replacement for same symbol used to represent a zero. The "∅" symbol is always drawn as a slashed circle, whereas in most typefaces the letter "Ø" is a slashed ellipse.
The diameter symbol (⌀) (Unicode character U+2300) is similar to the lowercase letter ø, and in some typefaces it even uses the same glyph, although in many others the glyphs are subtly distinguishable. The diameter symbol is used extensively in engineering drawings, and it is also seen anywhere that abbreviating "diameter" is useful, such as on camera lenses. For example, a lens with a diameter of 82 mm would be labeled ⌀ 82 mm.
There are at least two theories about the origin of the letter ø:
It possibly arose as a version of the ligature, Œ, of the digraph "oe", with the horizontal line of the "e" written across the "o".
That it arose in Anglo-Saxon England as an O and an I written in the same place, to represent a long close /ø/ sound resulting from i-mutation of /o/: compare Bede's Northumbrian Anglo-Saxon period spelling Coinualch for standard Cēnwealh (a man's name) (in a text in Latin). Later the letter ø disappeared from Anglo-Saxon as the Anglo-Saxon sound /ø/ changed to /e/, but by then use of the letter ø had spread from England to Scandinavia.
In OS X, it can be typed by holding O, or o, and then typing 6. In OS X and earlier systems, using a US English-language keyboard, the letter can be typed by holding the [Option] key while typing O, or o, to yield Ø, or ø.
On an Amiga operating system using any keyboard map, the letter can be typed by holding the [Alt] key while typing O, or o, to yield Ø, or ø.
On Microsoft Windows, using the "United States-International" keyboard setting, it can be typed by holding down the Alt-Gr key and pressing "L". It can also be typed under any keyboard setting by holding down the Alt key while typing 0216 (for uppercase) or 0248 (for lowercase) on the numeric keypad, provided the system uses code page 1252 as system default. (Code page 1252 is a superset of ISO 8859-1, and 216 and 248 are the decimal equivalents of hexadecimal D8 and F8.)
In the X Window System environment, one can produce these characters by pressing Alt-Gr and o or O, or by pressing the Multi key followed with a slash and then o or O.
In some systems, such as older versions of MS-DOS, the letter Ø is not part of the widely used code page 437. In Scandinavian codepages, Ø replaces the yen sign (¥) at 165, and ø replaces the ¢ sign at 162.
In Unicode, Ǿ and ǿ have the code points U+01FE and U+01FF.