O

In English, the name of the letter is the "long O" sound, pronounced /ˈoʊ/. In most other languages, its name matches the letter's pronunciation in open syllables.

Its graphic form has remained fairly constant from Phoenician times until today. The name of the Phoenician letter was ʿeyn, meaning "eye", and its shape originates simply as a drawing of a human eye (possibly inspired by the corresponding Egyptian hieroglyph, cf. Proto-Sinaitic script). Its original sound value was that of a consonant, probably [ʕ], the sound represented by the cognate Arabic letter ع ʿayn.^[2]

The use of this Phoenician letter for a vowel sound is due to the early Greek alphabets, which adopted the letter "omicron" to represent the vowel /o/. The letter was adopted with the value in the Old Italic alphabets, including the early Latin alphabet. In Greek, a variation of the form later came to differentiate this long sound (omega, meaning "large O") from the short o (Omicron, meaning "small o"). The Greek omicron gave rise to the corresponding Cyrillic letter O.^[2][3]

The letter ⟨o⟩ is the fourth most common letter in the English alphabet.^[4] Like the other English vowel letters, it has associated "long" and "short" pronunciations. The "long" ⟨o⟩ as in boat is actually most often a diphthong /oʊ/ (realized dialectically anywhere from [o]to[əʊ]). In English, there is also a "short" ⟨o⟩ as in fox, /ɒ/, which sounds slightly different in different dialects. 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 [ɑ] to a central vowel [a].^[5]

Common digraphs include ⟨oo⟩, which represents either /uː/or/ʊ/; ⟨oi⟩or⟨oy⟩, which typically represents the diphthong /ɔɪ/, and ⟨ao⟩, ⟨oe⟩, and ⟨ou⟩ which represent a variety of pronunciations depending on context and etymology.^[5]

In other contexts, especially before a letter with a minim, ⟨o⟩ may represent the sound /ʌ/, as in 'son' or 'love'. It can also represent the semivowel /w/, as in choirorquinoa.^{[citation needed]}

"O" in isolation is a word, also spelled "oh" and pronounced /oʊ/. Before a noun, usually capitalized, it indicates direct address (the vocative case), as in the titles "O Canada" or "O Captain! My Captain!" or in certain verses of the Bible.^[6]

⟨o⟩ is commonly associated with the open-mid back rounded vowel [ɔ], mid back rounded vowel [o̞]orclose-mid back rounded vowel [o] in many languages. 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.^{[citation needed]}

In the International Phonetic Alphabet, ⟨o⟩ represents the close-mid back rounded vowel.^[7]

• Oxygen, symbol O, a chemical element

• Ꝋ ꝋ : Forms of O were used for medieval scribal abbreviations^[9]
• ∅ : empty set symbol^[18]
• º : Masculine ordinal indicator
• Calligraphic O (𝒪, 𝓸): Mathematical Alphanumeric Symbols^[19]

• 𐤏 : Semitic letter Ayin, from which the following symbols originally derive:
  • Ω ω : Greek letter Omega
  • Ο ο : Greek letter Omicron
    • Ⲟ ⲟ : Coptic letter O, which derives from Greek omicron
    • О о : Cyrillic letter O, which also derives from Omicron
    • 𐌏 : Old Italic O, which derives from Greek Omicron, and is the ancestor of modern Latin O
    • Օ օ : Armenian letter O^{[citation needed]}

¹ Also for encodings based on ASCII, including the DOS, Windows, ISO-8859 and Macintosh families of encodings.^{[citation needed]}

• O mark
• Open O (Ɔ ɔ)
• 0 (zero). The capital letter O may be mistaken or misused for the number 0, as they appear quite identical in some typefaces. Early typewriters did not have a 'zero' key.

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