Its graphic form has also remained fairly constant from Phoenician times until today. The name of the Phoenician letter was ʿeyn, meaning "eye", and indeed its shape originates simply as a drawing of a human eye (possibly inspired by the corresponding Egyptian hieroglyph, c.f. Proto-Sinaitic script). Its original sound value was that of a consonant, probably [ʕ], the sound represented by the cognate Arabic letter letter ع ʿayn.
The use of this Phoenician letter for a vowel sound is due to the early Greek alphabets, which adopted the letter as O "omicron" to represent the vowel /o/. The letter was adopted with this value in the Old Italic alphabets, including the early Latin alphabet. In Greek, a variation of the form later came to distinguish this long sound (Omega, meaning "large O") from the short o (Omicron, meaning "small o"). Greek omicron gave rise to the corresponding Cyrillic letter O and the early Italic letter to runic runic ᛟ.
Even alphabets constructed "from scratch", i.e. not derived from Semitic, usually have similar forms to represent this sound, e.g.; the creators of the Afaka and Ol Chiki scripts, each invented in different parts of the world in the last century, both attributed their vowels for 'O' to the shape of the mouth when making this sound.[original research?]
The letter O is the fourth most common letter in the English language. O is most commonly associated with the Open-mid back rounded vowel [ɔ], Mid back rounded vowel [o̞] or close-mid back rounded vowel [o] in many languages. This form is colloquially termed the "long o" as in boat in English, but it is actually most often a diphthong /oʊ/ (realized dialectically anywhere from [o] to [əʊ]). In English there is a "short O" as in fox, which also has several pronunciations. In most dialects of British English, it is either an open-mid back rounded vowel [ɔ] or an open back rounded vowel [ɒ]; in American English, it is most commonly an unrounded back to a central vowel [ɑː] to [a].
Common digraphs include 'oo', which represents either /uː/, /ʊ/ or /ʌ/; 'oi' which typically represents the diphthong /ɔɪ/, like the pronunciation of 'oi' in "boil"; and 'ao', 'oe', and 'ou' which represent a variety of pronunciations depending on context and etymology.
Other languages use O for various values, usually back vowels which are at least partly open. Derived letters such as Ö and Ø have been created for the alphabets of some languages to distinguish values that were not present in Latin and Greek, particularly rounded front vowels.
Related letters and other similar characters 
- Ο ο : Greek letter Omicron
- О о : Cyrillic letter O
- Օ օ : Armenian letter O
- 0 : digit zero, which is often confused with the letter O
- Ø ø : Latin letter Ø
- ∅ : empty set symbol
- ⌀ : diameter symbol
- ° : degree symbol
- º : masculine ordinal indicator (in Galician, Italian, Portuguese, and Spanish)
Computing codes 
|Unicode name||LATIN CAPITAL LETTER O||LATIN SMALL LETTER O|
|Numeric character reference||O||O||o||o|
- 1 Also for encodings based on ASCII, including the DOS, Windows, ISO-8859 and Macintosh families of encodings.
Other representations 
- "O" Oxford English Dictionary, 2nd edition (1989); Chambers-Happap, "oes" op. cit. Oes is the plural of the name of the letter. The plural of the letter itself is rendered Os, O's, os, o's.
- Media related to O at Wikimedia Commons
- The dictionary definition of O at Wiktionary
- The dictionary definition of o at Wiktionary
Letter O with diacritics