O (named o /ˈoʊ/, plural oes) is the fifteenth letter and a vowel in the ISO basic Latin alphabet. The letter was derived from the Semitic `Ayin (eye), which represented a consonant, probably, the sound represented by the Arabic letter ع called `Ayn. This Semitic letter in its original form seems to have been inspired by a similar Egyptian hieroglyph for "eye". The Greeks are thought to have come up with the innovation of vowel characters, and lacking a pharyngeal consonant, employed this letter as the Greek O to represent the vowel /o/, a sound it maintained in Etruscan and Latin. In Greek, a variation of the form later came to distinguish this long sound (Omega, meaning "large O") from the short o (Omicron, meaning "small o").

Its graphic form has also remained fairly constant from Phoenician times until today. Indeed, even alphabets constructed "from scratch", i.e. not derived from Semitic, usually have similar forms to represent this sound, e.g.; the creators of the Afaka and Ol Chiki scripts, each invented in different parts of the world in the last century, both attributed their vowels for 'O' to the shape of the mouth when making this sound.

Read more about O:  Usage, Related Letters and Other Similar Characters, Computing Codes, Other Representations