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O, o

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noun, plural O's or Os; o's or os or oes.
  1. the fifteenth letter of the English alphabet, a vowel.
  2. any spoken sound represented by the letter O or o, as in box, note, short, or love.
  3. something having the shape of an O.
  4. a written or printed representation of the letter O or o.
  5. a device, as a printer's type, for reproducing the letter O or o.


  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.
  1. the exclamation “O.”

Origin of O

1125–75; Middle English < Old French < Latin ō
Can be confusedO oh owe


  1. Old.
  2. Grammar. object.


  1. the fifteenth in order or in a series.
  2. the Arabic cipher; zero.
  3. (sometimes lowercase) the medieval Roman numeral for 11.Compare Roman numerals.
  4. Physiology. a major blood group, usually enabling a person whose blood is of this type to donate blood to persons of group O, A, B, or AB and to receive blood from persons of group O.Compare ABO system.
  5. Chemistry. oxygen.
  6. Logic. particular negative.


[uh, oh]
  1. an abbreviated form of of, as in o'clock or will-o'-the-wisp.
  2. an abbreviated form of on.

Origin of o'

Middle English; by shortening.


  1. a prefix meaning “descendant,” in Irish family names: O'Brien; O'Connor.

Origin of O'

representing Irish ó descendant, Old Irish au


  1. an abridgment of ortho-.


  1. variant of ob- before m: omission.


  1. variant of oo-: oidium.


  1. pint.

Origin of o.1

From the Latin word octārius


  1. octavo.
  2. off.
  3. old.
  4. only.
  5. order.
  6. Baseball. out; outs.


  1. a suffix occurring as the final element in informal shortenings of nouns (ammo; combo; condo; limo; promo); -o also forms nouns, usually derogatory, for persons or things exemplifying or associated with that specified by the base noun or adjective (cheapo; pinko; sicko; weirdo; wino).
  2. a suffix occurring in colloquial noun or adjective derivatives, usually grammatically isolated, as in address: cheerio; kiddo; neato; righto.

Origin of -o

perhaps orig. the interjection O, appended to words as in def 2; as a derivational suffix reinforced by clipped forms of words with -o- as a linking element (e.g., photo, stereo), by Rom nouns ending in o, and by personal nouns such as bimbo and bozo, of obscure origin


  1. the typical ending of the first element of compounds of Greek origin (as -i- is, in compounds of Latin origin), used regularly in forming new compounds with elements of Greek origin and often used in English as a connective irrespective of etymology: Franco-Italian; geography; seriocomic; speedometer.
Compare -i-.

Origin of -o-

Middle English (< Old French) < Latin < Greek


  1. (in prescriptions) a pint.

Origin of O.1

From the Latin word octārius


  1. Ocean.
  2. octavo.
  3. October.
  4. Ohio.
  5. Old.
  6. Ontario.
  7. Oregon.


[om-i-kron, oh-mi-]
  1. the 15th letter of the Greek alphabet (O, o).
  2. the vowel sound represented by this letter.

Origin of omicron

< Greek ō mikrón, literally, small o. Cf. omega
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for o



noun plural o's, O's or 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


symbol for
  1. chem oxygen
  2. a human blood type of the ABO groupSee universal donor
  3. logic a particular negative categorial proposition, such as some men are not married: often symbolized as SoPCompare A, E, I 2
abbreviation for
  1. Australian slang offence

Word Origin

(for sense 3) from Latin (neg) o I deny


  1. a variant spelling of oh
  2. an exclamation introducing an invocation, entreaty, wish, etcO God!; O for the wings of a dove!


  1. short for ortho- (def. 4)


  1. informal, or archaic shortened form of of a cup o' tea


  1. forming informal and slang variants and abbreviations, esp of nounswino; lie doggo; Jacko

Word Origin

probably special use of oh


  1. the 15th letter in the Greek alphabet (Ο, ο), a short vowel, transliterated as o

Word Origin

from Greek ō mikron small o; see micro-, omega


connective vowel
  1. used to connect elements in a compound wordchromosome; filmography Compare -i-

Word Origin

from Greek, stem vowel of many nouns and adjectives in combination
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for o

interjection of fear, surprise, admiration, etc.; see oh.


blood type, 1926, originally "zero," denoting absence of A and B agglutinogens.


as a prefix in Irish names, from Irish ó, ua (Old Irish au) "descendant."


15th letter of the Greek alphabet, literally "small 'o,' " from Greek (s)mikros "small," from PIE *smik-. Because the vowel was "short" in ancient Greek. Cf. Omega.

Online Etymology Dictionary, © 2010 Douglas Harper

o in Medicine


  1. ortho- (often italic)


  1. Used as a connective to join word elements:acidophilic.


(ŏmĭ-krŏn′, ōmĭ-)
  1. The 15th letter of the Greek alphabet.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

o in Science


  1. The symbol for oxygen.


  1. A nonmetallic element that exists in its free form as a colorless, odorless gas and makes up about 21 percent of the Earth's atmosphere. It is the most abundant element in the Earth's crust and occurs in many compounds, including water, carbon dioxide, and iron ore. Oxygen combines with most elements, is required for combustion, and is essential for life in most organisms. Atomic number 8; atomic weight 15.9994; melting point -218.8°C; boiling point -182.9°C; gas density at 0°C 1.429 grams per liter; valence 2. See Periodic Table.
Word History: In 1786, the French chemist Antoine Lavoisier coined a term for the element oxygen (oxygène in French). He used Greek words for the coinage: oxy- means “sharp,” and -gen means “producing.” Oxygen was called the “sharp-producing” element because it was thought to be essential for making acids. Lavoisier also coined the name of the element hydrogen, the “water-producing” element, in 1788. Soon after, in 1791, another French chemist, J. A. Chaptal, introduced the word nitrogen, the “niter-producing” element, referring to its discovery from an analysis of nitric acid.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.