O, or o, is the fifteenth letter and the fourth vowel letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is o (pronounced /ˈoʊ/), plural oes.[1]
History
Late Renaissance or early Baroque design of an O, from 1627

Its graphic form has 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, 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.[citation needed]

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 ᛟ.[citation needed]

Even 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.[original research?]

Use in writing systems
English

The letter ⟨o⟩ is the fourth most common letter in the English alphabet.[2] 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].[citation needed]

Common digraphs include ⟨oo⟩, which represents either /u:/ 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.[citation needed]

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 choir or quinoa.[citation needed]

In English, the letter ⟨o⟩ in isolation before a noun, usually capitalized, marks the vocative case, as in the titles to O Canada or O Captain! My Captain! or certain verses of the Bible.[3]
Other languages
Pronunciation of the name of the letter ⟨o⟩ in European languages

⟨o⟩ is 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. 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]
Other systems

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

Related characters
See also: circle symbol
Descendants and related characters in the Latin alphabet

Œ œ : Latin OE ligature
O with diacritics: Ø ø Ǿ ǿ ᶱ[4] Ö ö Ȫ ȫ Ó ó Ò ò Ô ô Ố ố Ồ ồ Ổ ổ Ỗ ỗ Ộ ộ Ǒ ǒ Ő ő Ŏ ŏ Ȏ ȏ Ȯ ȯ Ȱ ȱ Ọ ọ Ɵ ɵ Ơ ơ Ớ ớ Ờ ờ Ỡ ỡ Ợ ợ Ở ở Ỏ ỏ Ō ō Ṓ ṓ Ṑ ṑ Õ õ Ȭ ȭ Ṍ ṍ Ṏ ṏ Ǫ ǫ Ȍ ȍ O̩ o̩ Ó̩ ó̩ Ò̩ ò̩ Ǭ ǭ O͍ o͍
Ꝍ ꝍ : O with loop was used in some medieval Nordic orthographies[5]
Ꟁ ꟁ : Old Polish O[6]
ⱺ : Small o with low ring inside is used in the Swedish Dialect Alphabet[7]
IPA-specific symbols related to O: ɔ
IPA superscript letters:[8] 𐞢 𐞣
Uralic Phonetic Alphabet-specific symbols related to O:[9]
U+1D0F ᴏ LATIN LETTER SMALL CAPITAL O
U+1D3C ᴼ MODIFIER LETTER CAPITAL O
U+1D52 ᵒ MODIFIER LETTER SMALL O
U+1D11 ᴑ LATIN SMALL LETTER SIDEWAYS O
U+1D13 ᴓ LATIN SMALL LETTER SIDEWAYS O WITH STROKE
U+1D16 ᴖ LATIN SMALL LETTER TOP HALF O
U+1D17 ᴗ LATIN SMALL LETTER BOTTOM HALF O
U+1D54 ᵔ MODIFIER LETTER SMALL TOP HALF O
U+1D55 ᵕ MODIFIER LETTER SMALL BOTTOM HALF O
Teuthonista phonetic transcription-specific symbols related to O:[10]
U+AB3D ꬽ LATIN SMALL LETTER BLACKLETTER O
U+AB3E ꬾ LATIN SMALL LETTER BLACKLETTER O WITH STROKE
U+AB3F ꬿ LATIN SMALL LETTER OPEN O WITH STROKE
U+AB43 ꭃ LATIN SMALL LETTER TURNED O OPEN-O
U+AB44 ꭄ LATIN SMALL LETTER TURNED O OPEN-O WITH STROKE
o : Subscript small o is used in Indo-European studies[11]
𝼛 : Latin small letter o with retroflex hook is used in phonetic transcription[12][13]

Derived signs, symbols and abbreviations

Ꝋ ꝋ : Forms of O were used for medieval scribal abbreviations[5]
∅ : empty set symbol[14]
º : Masculine ordinal indicator
Calligraphic O (𝒪, 𝓸): Mathematical Alphanumeric Symbols[citation needed]

Ancestors and siblings in other alphabets

𐤏 : Semitic letter Ayin, from which the following symbols originally derive
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]