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.
      [1913 Webster]

   3. A cipher; zero. [R.]
      [1913 Webster]

            Thou art an O without a figure.       --Shak.
      [1913 Webster]

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.
   [1913 Webster]

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]

WordNet (r) 3.0 (2006):

os
    n 1: a mouth or mouthlike opening
    2: a hard brittle blue-grey or blue-black metallic element that
       is one of the platinum metals; the heaviest metal known [syn:
       osmium, Os, atomic number 76]
    3: (computer science) software that controls the execution of
       computer programs and may provide various services [syn:
       operating system, OS]
    4: the left eye [syn: oculus sinister, OS]
    5: rigid connective tissue that makes up the skeleton of
       vertebrates [syn: bone, os]

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

OS
       Operating System


The Jargon File (version 4.4.7, 29 Dec 2003):

OS
 /O?S/

    1. [Operating System] n. An abbreviation heavily used in email,
    occasionally in speech.

    2. n. obs. On ITS, an output spy. See OS and JEDGAR in Appendix A.


The Free On-line Dictionary of Computing (30 December 2018):

OS

   1. operating system.

   2. [obsolete, ITS], an output spy.  See "OS and JEDGAR".

   3.  An operating system from IBM for
   their System/360 line of hardware announced in 1964.  OS was
   planned with several flavours that were supposed to be
   compatible.  OS was late, memory hungry and not able to reach
   the marketing objectives of IBM for the 360/30, the planned
   successor of the IBM 1401.  IBM then decided to design a new
   operating system for the low end machines which they called
   DOS/360.

   [Jargon File]

   (1997-09-22)