1. [n]  the 15th letter of the Roman alphabet
2. [n]  the blood group whose red cells carry neither the A nor B antigens; "people with type O blood are universal donors"
3. [n]  a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust

1. \O\ ([=o]).
   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
      Ph[oe]nician, 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. b[=a]n; E.
      stone, AS. st[=a]n; E. broke, AS. brecan to break; E.
      bore, AS. beran to bear; E. dove, AS. d[=u]fe; E. toft,
      tuft; tone, tune; number, F. nombre. The letter o has
      several vowel sounds, the principal of which are its long
      sound, as in bone, its short sound, as in nod, and the
      sounds heard in the words orb, son, do (feod), and wolf
      (book). In connection with the other vowels it forms
      several digraphs and diphthongs. See Guide to
      Pronunciation, [sect][sect] 107-129.
   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. O was also anciently used to represent 11:
      with a dash over it ([=O]), 11,000.
   

2. \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.
   

3. \O'\ [Ir. o a descendant.]
   A prefix to Irish family names, which signifies grandson or
   descendant of, and is a character of dignity; as, O'Neil,
   O'Carrol.
   

4. \O'\ ([=o]; unaccented [-o]), prep.
   A shortened form of of or on. ``At the turning o' the tide.''
   --Shak.
   

5. \O\ ([=o]), a. [See {One}.]
   One. [Obs.] --Chaucer. ``Alle thre but o God.'' --Piers
   Plowman.
   

6. \O\, interj.
   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. --Ps.
                                                  cxix. 89.
         O how love I thy law ! it is my meditation all the day.
                                                  --Ps. cxix.
                                                  97.
   Note: O is frequently followed by an ellipsis and that, an in
         expressing a wish: ``O [I wish] that Ishmael might live
         before thee !'' --Gen. xvii. 18; or in expressions of
         surprise, indignation, or regret: ``O [it is sad] that
         such eyes should e'er meet other object !'' --Sheridan
         Knowles.
   Note: A distinction between the use of O and oh is insisted
         upon by some, namely, that O should be used only in
         direct address to a person or personified object, and
         should never be followed by the exclamation point,
         while Oh (or oh) should be used in exclamations where
         no direct appeal or address to an object is made, and
         may be followed by the exclamation point or not,
         according to the nature or construction of the
         sentence. Some insist that oh should be used only as an
         interjection expressing strong feeling. The form O,
         however, is, it seems, the one most commonly employed
         for both uses by modern writers and correctors for the
         press. ``O, I am slain !'' --Shak. ``O what a fair and
         ministering angel !'' ``O sweet angel !'' --Longfellow.
               O for a kindling touch from that pure flame !
                                                  --Wordsworth.
               But she is in her grave, -- and oh The difference
               to me !                            --Wordsworth.
               Oh for a lodge in some vast wilderness !
                                                  --Cowper.
               We should distinguish between the sign of the
               vocative and the emotional interjection, writing
               O for the former, and oh for the latter. --Earle.
   {O dear}, & {O dear me!} [corrupted fr. F. O Dieu! or It. O
      Dio! O God! O Dio mio! O my God! --Wyman.], exclamations
      expressive of various emotions, but usually promoted by
      surprise, consternation, grief, pain, etc.
   

ASCII code 79, The letter of the alphabet, not to be confused with 0 (zero) the digit.