OS.

O

[oh]
interjection
1.
(used before a name in direct address, especially in solemn or poetic language, to lend earnestness to an appeal): Hear, O Israel!
2.
(used as an expression of surprise, pain, annoyance, longing, gladness, etc.)
noun, plural O's.
3.
the exclamation “O.”

Origin:
1125–75; Middle English < Old French < Latin ō

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
o or O (əʊ)
 
n , pl o's, O's, Os
1.  the 15th letter and fourth vowel of the modern English alphabet
2.  any of several speech sounds represented by this letter, in English as in code, pot, cow, move, or form
3.  another name for nought
 
O or O
 
n

O1
 
symbol for
1.  chem oxygen
2.  See universal donor a human blood type of the ABO group
3.  logic A E Compare I a particular negative categorial proposition, such as some men are not married: often symbolized as SoP
 
abbreviation for
4.  slang (Austral) offence
 
[(for sense 3) from Latin (neg)o I deny]

O2 (əʊ)
 
interj
1.  a variant spelling of oh
2.  an exclamation introducing an invocation, entreaty, wish, etc: O God!; O for the wings of a dove!

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Etymonline
Word Origin & History

o
interj. of fear, surprise, admiration, etc., see oh.

O
blood type, 1926, originally "zero," denoting absence of A and B agglutinogens.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

o
The Greek letter omicron. Entries beginning with this character are alphabetized under omicron.

O 1

The symbol for the element oxygen.

O 2
abbr.
oculus

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
O  
The symbol for oxygen.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Abbreviations & Acronyms
o
  1. Latin octarius (pint)

  2. octavo

  3. ohm

O
  1. old

  2. Oriental (as in personal ads, but usually A, Asian)

  3. out

  4. outstanding

The American Heritage® Abbreviations Dictionary, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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