{{for|the Underoath album|Ø (Disambiguation)}} {{wiktionary|-o|o|O|o-}} '''O''', or '''o''', is the fifteenth letter of the English alphabet.

'''O''' may also refer to: {{TOC right}}

== Letters == * {{unichar|0555|nlink=}}, {{unichar|0585|nlink=}}, letters in the Armenian alphabet * {{unichar|10ff|nlink=}}, a letter in the Georgian 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 * {{unichar2|ㅇ|nlink=}}, a consonant in Hangul, the Korean alphabet * {{unichar2|ဝ|nlink=}}, a consonant in Burmese script

==Arts and entertainment== ===Film and television=== * ''O'' (2001 film), a film starring Mekhi Phifer, Josh Hartnett, and Julia Stiles * ''O'' (2022 film), an Indian Kannada-language horror thriller film

===Literature=== * ''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=== * ''O, The Oprah Magazine'', magazine founded by Oprah Winfrey * ''O'' (German magazine), a fetish magazine

===Music=== * "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'' (Oval album), 2010 * ''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), nickname of the American musician Otis Barthoulameu * A Band Called O

===Sports=== * 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=== * ''O'' (Cirque du Soleil), show at the Bellagio hotel and casino in Las Vegas, Nevada

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

==Linguistics== * /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== * <math>f = O(g)</math>, a Bachmann–Landau notation in computational complexity theory ("|''f''| is bounded above by ''g'' asymptotically") * <math>f = o(g)</math>, 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 <math>\mathcal{O}</math>, a category of Lie algebra representations * o, for octet, an information measure unit used in computing * Kleene's O, a system of ordinal notations

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

==Places== * O Brook, a short river in Devon, England * O'Block, a colloquial name for the Parkway Garden Homes in Chicago

== Science == * 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 * O-type star, stellar classification for very hot bluish stars * O (for ''Octavius'', meaning an eighth of a gallon), an apothecary's symbol for a pint

==Surname== * 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== * 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 * 17px|O, the official West Japan Railway Company service symbol for the Osaka Loop Line

==See also== {{canned search|o}} * {{srt|O}} * Mr. O (TV show) * Circle (disambiguation) * Circle symbol (disambiguation) * Zero (disambiguation) * Big O (disambiguation) * Oh (disambiguation) * Omicron (disambiguation) including Ο/ο

{{disambiguation}}