In most dialects of British English, it is either an open-mid back rounded vowel or an open back rounded vowel in American English, it is most commonly an unrounded back to a central vowel. In English there is also a 'short' ⟨o⟩ as in fox, / ɒ/, which sounds slightly different in different dialects. The 'long' ⟨o⟩ as in boat is actually most often a diphthong / oʊ/ (realized dialectically anywhere from to ). Like the other English vowel letters, it has associated 'long' and 'short' pronunciations. The letter ⟨o⟩ is the fourth most common letter in the English alphabet. Įven alphabets that are not derived from Semitic tend to have similar forms to represent this sound for example, 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. Greek omicron gave rise to the corresponding Cyrillic letter O and the early Italic letter to runic ᛟ. 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').

The letter was adopted with this value in the Old Italic alphabets, including the early Latin alphabet. 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/.

Its original sound value was that of a consonant, probably, the sound represented by the cognate Arabic letter ع ʿayn. ʿeyn, meaning 'eye', and indeed its shape originates simply as a drawing of a human eye (possibly inspired by the corresponding Egyptian hieroglyph, cf. Its graphic form has remained fairly constant from Phoenician times until today.

Late Renaissance or early Baroque design of an O, from 1627

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