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The letter ⟨o⟩ is the fourth most common letter in the English alphabet.^[3] Like the other English vowel letters, it has associated "long" and "short" pronunciations. The "long" ⟨o⟩ as in both 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].^[4]

Common digraphs include ⟨oo⟩, which represents either /uː/, /ʊ/ or /ʌ/; ⟨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.^[4]

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.^[5]

"O" in isolation is a word, also spelled "oh" and pronounced /oʊ/. Before a noun, usually capitalized, it indicates a noun of address, 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] 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.^[7]

In the International Phonetic Alphabet, ⟨o⟩ represents the close-mid back rounded vowel.^[8]

• U+1DF72 𝽲 LATIN CAPITAL LETTER O WITH CURL and U+1DF73 𝽳 LATIN SMALL LETTER O WITH CURL : O with curl is used in the English Phonotypic Alphabet^[20]

• Ꝋ ꝋ : Forms of O were used for medieval scribal abbreviations^[10]
• ∅ : empty set symbol^[21]
• º : Masculine ordinal indicator
• Calligraphic O (𝒪, 𝓸): Mathematical Alphanumeric Symbols^[22]

• 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

• U+004F O LATIN CAPITAL LETTER O
• U+006F o LATIN SMALL LETTER O
• U+FF2F Ｏ FULLWIDTH LATIN CAPITAL LETTER O
• U+FF4F ｏ FULLWIDTH LATIN SMALL LETTER O