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').

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 châteaux in France, e.g. Château d'O in Normandy [1]
• in French, a homophone of eau (water)
• a theatrical production in Las Vegas. "O" is a Cirque du Soleil production, a theatrical acrobatic production set in a water stage.
• 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.
• O is 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 dot in the middle or a line through it like Ø or Θ).
• O is one of the human blood types.
• the subject of the sadomasochistic novel The Story of O

Two-letter combinations starting with O:

• oa ob oc od oe of og oh oi oj ok ol om on oo op oq or os ot ou ov ow ox oy oz