The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. Something shaped like the letter O; a circle or oval.
      "This wooden O [Globe Theater]". --Shak.
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   3. A cipher; zero. [R.]
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            Thou art an O without a figure.       --Shak.
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The Collaborative International Dictionary of English v.0.48:

Os \Os\, n.; pl. Ossa. [L.]
   A bone.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Os \Os\, n.; pl. Ora. [L.]
   A mouth; an opening; an entrance.
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The Collaborative International Dictionary of English v.0.48:

Os \Os\, n.; pl. Osar. [Sw. [*a]s ridge, chain of hills, pl.
   [*a]sar.] (Geol.)
   One of the ridges of sand or gravel found in Sweden, etc.,
   supposed by some to be of marine origin, but probably formed
   by subglacial waters. The osar are similar to the kames of
   Scotland and the eschars of Ireland. See Eschar.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Os \Os\, n. (Chem.)
   The chemical symbol for the element osmium.
   [PJC]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

OS
       Operating System