O

O is the fifteenth letter of the Latin alphabet and a vowel used in many languages. In English its name is commonly pronounced as "oh," and it appears in both uppercase O and lowercase o. The letter is visually a circular or nearly circular shape.

Etymology and history

The Latin O derives from the Greek letter omicron, which means "small o" to distinguish it from omega, meaning "large o." The form was transmitted to the Latin alphabet through the Etruscans and later adapted into modern alphabets used today.

Linguistic and orthographic use

As a vowel, O represents a range of vocalic sounds across languages, with pronunciation varying by dialect and word. Many languages employ diacritics with O to mark distinct sounds, such as ó, ö, õ, ō, or ô, reflecting diverse phonologies. The letter also appears in digraphs and loanwords and may serve as a grammatical or phonetic marker in some writing systems.

Scientific and mathematical usage

In science, O is the chemical symbol for oxygen, the third most abundant element in the universe and essential to most forms of life. Its diatomic form is O2. In mathematics and computer science, Big-O notation describes an upper bound on growth or complexity, as in O(n). The orthogonal group, denoted O(n), comprises rotations and reflections preserving distances in n-dimensional space.

Other contexts

In astronomy, spectral class O denotes very hot, blue stars. In typography and digital text, O and o have specific Unicode code points, U+004F for uppercase and U+006F for lowercase. The letter also appears as an interjection in some historical texts and as a basic symbol of a circle in various symbolic contexts.