O is the fifteenth letter of the Latin alphabet.

In Greek (Omikron), Etruscan and Latin O stood for the vowel /o/. Although Semitic 'Ajin was used in some alphabets to transcribe [o], the sound value was usually consonantic: [?/] (as the Arabic letter ع called 'Ajn').

A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z

Oscar represents the letter O in the NATO phonetic alphabet.

O is also:

  • used in complexity theory and mathematics to describe the size of functions, using a notation called "Big O notation".
  • the symbol for oxygen in the periodic table of the elements.
  • the name of several chteaux in France, e.g. Chteau d'O in Normandy [1]
  • in French, a homophone of eau (water)
  • a movie based on William Shakespeare's Othello and set in a high school. The duke is the basketball coach nicknamed "Duke", and the Othello role is a basketball star nicknamed "O". The movie is a very realistic portrayal of how adolescent jealousy can get out of hand, leading to deadly consequences.
  • the name of a major magazine publication founded and edited by entrepreneur Oprah Winfrey.
  • the stock symbol for Realty Income Corporation.
  • the symbol for nought or zero (on a computer keyboard 0 the number is more oblong shaped than O the letter. Sometimes 0 the number has a line through it like Θ but thinner).
  • O is one of the Blood types.
  • the subject of the sadomasochistic novel The Story of O

See also: , , , , Ŏ, , (Oe), , (OE), Ő

Two-letter combinations starting with O: