O, or o, is the fifteenth letter of the English alphabet.

O may also refer to:

Letters[edit]

• Օ օ, (Unicode: U+0555, U+0585) a letter in the Armenian alphabet
• Ο ο, Omicron, (Greek), a letter in the Greek alphabet
• O (Cyrillic), a letter of the Cyrillic alphabet
• O (kana), a romanization of the kana (お and オ) in Japanese writing
• ㅇ, a consonant in Hangul, the Korean alphabet
• ဝ, a consonant in Burmese script

Arts and entertainment[edit]

Film and television[edit]

• O (film), 2001 film starring Josh Hartnett, Mekhi Phifer and Julia Stiles

Literature[edit]

• O: A Presidential Novel, anonymous novel published in 2011
• O, fictional planet that is the setting of several short stories by science fiction author Ursula K. Le Guin
• O, fictional character from the French erotic novel Story of O
• "O" Is for Outlaw, the fifteenth novel in Sue Grafton's "Alphabet mystery" series, published in 1999

Media and publications[edit]

• O, The Oprah Magazine, magazine founded by Oprah Winfrey
• O (German magazine), a fetish magazine

Music[edit]

• "O (Oh!)", 1920 by Ted Lewis, 1953 by Pee Wee Hunt
• O (A.C. Acoustics album), 2002
• O (Damien Rice album), 2002
• O (Eiko Shimamiya album), 2006
• O (Omarion album), 2005
  • "O" (Omarion song), the title song
• o (Tilly and the Wall album), 2008
• O (EP), a 2023 EP by Yuju
• O, 2002 album by Zone
• "O", a 2014 song by Coldplay from Ghost Stories
• "o", a song by iamamiwhoami
• "O!", a song by the American band Bright from The Albatross Guest House, 1997
• O (musician), American musician

Sports[edit]

• O (gesture), a gesture used to show support for the sports teams of the University of Oregon
• Baltimore Orioles, an American Major League Baseball team, among whose common nicknames are "O's" and "The O's"

Others[edit]

• O (Cirque du Soleil), show at the Bellagio hotel and casino in Las Vegas, Nevada

Companies[edit]

• Essie's Original Hot Dog shop (commonly known as "The O"), a restaurant in Pittsburgh, Pennsylvania
• Overstock.com, online retailer previously known as O.co
• Realty Income, American real estate investment trust (NYSE stock ticker O)

History[edit]

• O-class battlecruiser, class of German ships in the 1930s
• The O (political group) (The Organization), a 1970s American political group

Linguistics[edit]

• /o/, close-mid back rounded vowel in the International Phonetic Alphabet
• O, an antiquated vocative placed before a name of, or phrase characterizing, the party being addressed
• O (also P), the patient-like argument (object) of a canonical transitive verb
• O, gender neutral third person pronoun in the Turkish language
• O-, an honorific prefix used in the Japanese language

Mathematics[edit]

• , a Bachmann–Landau notation in computational complexity theory ("|f| is bounded above by g asymptotically")
• , a Bachmann–Landau notation in computational complexity theory ("f is dominated by g asymptotically")
• O, for orthogonal group
• ∘ indicates function composition, or composition of morphisms in a category
• Category O or , a category of Lie algebra representations
• o, for octet, an information measure unit used in computing

Media[edit]

• O, an IRC operator service in QuakeNet's IRC services
• O Channel, the former name of Moji a television network in Indonesia

Places[edit]

• O Brook, a short river in Devon, England

Science[edit]

• Oxygen, symbol O, a chemical element
• O, a human blood type, the universal donor
• Haplogroup O (Y-DNA), a Y-chromosomal DNA haplogroup
• O, for pyrrolysine an alpha amino acid in biochemistry
• Class O, stellar classification for very hot bluish stars
• O (for Octavius, meaning an eight of a gallon), an apothecary's symbol for a pint

Surname[edit]

• O (surname), Korean surname romanized "O" or "Oh"
• Ō (Japanese surname), Japanese surname derived from the Chinese surname Wang (王)
• Ō clan a historical Japanese clan with an unrelated surname
• Ou (surname), Chinese and Cantonese surnames pronounced "O" or "Oh"
• A prefix meaning "grandson (of)", appearing in Irish surnames
• Genovevo de la O (1876-1952), Mexican revolutionary
• Karen O, born Karen Lee Orzolek, American singer

Other uses[edit]

• Oreo O's, an American breakfast cereal available internationally
• Oscar, the military time zone code for UTC−02:00: List of military time zones#O
• , the official West Japan Railway Company service symbol for the Osaka Loop Line.

See also[edit]

• All pages with titles beginning with O
• All pages with titles containing O
• Mr. O (TV show)
• Circle (disambiguation)
• Circle symbol (disambiguation)
• Zero (disambiguation)
• Big O (disambiguation)
• Omicron (disambiguation) including Ο/ο