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 
- 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
Two-letter combinations starting with O: