O (disambiguation)

O is the fifteenth letter of the modern Latin alphabet.

O may also refer to:

Science and mathematics

Haplogroup O (Y-DNA), a Y-chromosomal DNA haplogroup
Big O notation, used to describe the asymptotic behavior of a function or algorithm
O, chemical symbol for Oxygen
O, a human blood type, the universal donor
O, for Orthogonal group
o indicates function composition or composition of morphisms in a category
O, for pyrrolysine an alpha amino acid in biochemistry
$\mathcal{O}$ , a category of Lie algebra representations
o, for octet, an information measure unit used in computing

O (Damien Rice album), 2002 album by Damien Rice
O (Eiko Shimamiya album), 2006 album by Eiko Shimamiya
O (Omarion album), 2005 album by Omarion
o (Tilly and the Wall album), 2008 album by Tilly and the Wall
O (film), 2001 film starring Josh Hartnett, Mehki Phifer and Julia Stiles
O (Cirque du Soleil), show at the Bellagio hotel and casino in Las Vegas, Nevada
O, 2002 album by Zone
O, fictional character from the French erotic novel Story of O
O, fictional planet that is the setting of several short stories by science fiction author Ursula K. Le Guin
o, an iamamiwhoami song
O: A Presidential Novel, anonymous novel published in 2011
O: The Oprah Magazine, magazine founded by Oprah Winfrey
«O», an international fetish magazine

O (Cyrillic), a letter of the Cyrillic alphabet
O (kana), a romanization of the kana (お and オ) in Japanese writing
ㅇ in Hangul, Korean alphabet
/o/, the International Phonetic Alphabet symbol for a close-mid back rounded vowel
O (surname), a Chinese and Japanese family name
O-class battlecruiser, class of German ships in the 1930s
Class O, stellar classification for very hot bluish stars
The O (political group) (The Organization), a 1970s American political group
O Brook, a short river in Devon, England
Essie's Original Hot Dog shop (commonly known as "The O"), a restaurant in Pittsburgh, Pennsylvania
O (vocative), placed before a name of, or phrase characterizing, the party being addressed in speech