# O (disambiguation)

O, or o, is the fifteenth letter of the English alphabet.
O may also refer to:

## Letters

* 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
### Film and television

* O (2001 film), a film starring Josh Hartnett, Mekhi Phifer 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 (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

### 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

* $f=O(g)$, a Bachmann–Landau notation in computational complexity theory ("|f| is bounded above by g asymptotically")
* $f=o(g)$, 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 ${\mathcal {O}}$, a category of Lie algebra representations
* o, for octet, an information measure unit used in computing

## 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

## 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
* 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

* 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
* , the official West Japan Railway Company service symbol for the Osaka Loop Line.