Source : Webster's Revised Unabridged Dictionary (1913)

O \O\ ([=o]), n.; pl. {O's} or {Oes} ([=o]z).
   1. The letter O, or its sound. ``Mouthing out his hollow oes
      and aes.'' --Tennyson.

   2. Something shaped like the letter O; a circle or oval.
      ``This wooden O [Globe Theater]''. --Shak.

   3. A cipher; zero. [R.]

            Thou art an O without a figure.       --Shak.