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.”

  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.


  1. an abbreviated form of of, as in o’clock or will-o’-the-wisp.
  2. an abbreviated form of on.

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


  1. an abridgment of ortho-.

  1. variant of ob- before m: omission.

  1. variant of oo-: oidium.

  1. pint.

  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.

  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.

  1. (in prescriptions) a pint.

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


  1. the 15th letter of the Greek alphabet (O, o).
  2. the vowel sound represented by this letter.

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


  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


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

connective vowel

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

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. abbr.

  1. ortho- (often italic)

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


  1. The 15th letter of the Greek alphabet.

  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.

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