O /(ō)/

1. O, the fifteenth letter of the English alphabet, derives its form, value, and name from the Greek O, through the Latin. The letter came into the Greek from the Phœnician, which possibly derived it ultimately from the Egyptian. Etymologically, the letter o is most closely related to a, e, and u; as in E. bone, AS. bān; E. stone, AS. stān; E. broke, AS. brecan to break; E. bore, AS. beran to bear; E. dove, AS. dūfe; E. toft, tuft; tone, tune; number, F. nombre.
2. Among the ancients, O was a mark of triple time, from the notion that the ternary, or number 3, is the most perfect of numbers, and properly expressed by a circle, the most perfect figure.

pl. O's, Oes ((ōz))

1. The letter O, or its sound.
2. Something shaped like the letter O; a circle or oval.
3. A cipher; zero. [R.]

   Thou art an O without a figure.

1. A prefix to Irish family names, which signifies grandson or descendant of, and is a character of dignity; as, O'Neil, O'Carrol.

1. A shortened form of of or on.

1. One. [Obs.]

1. An exclamation used in calling or directly addressing a person or personified object; also, as an emotional or impassioned exclamation expressing pain, grief, surprise, desire, fear, etc.

   For ever, O Lord, thy word is settled in heaven.

   O how love I thy law ! it is my meditation all the day.

   O for a kindling touch from that pure flame!

   But she is in her grave, -- and oh The difference to me!

   Oh for a lodge in some vast wilderness!

   We should distinguish between the sign of the vocative and the emotional interjection, writing O for the former, and oh for the latter.