O, o

[ oh ]
/ oʊ /

noun, plural O's or Os; o's or os or oes.

the fifteenth letter of the English alphabet, a vowel.
any spoken sound represented by the letter O or o, as in box, note, short, or love.
something having the shape of an O.
a written or printed representation of the letter O or o.
a device, as a printer's type, for reproducing the letter O or o.



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Question 1 of 7

Definition for o (2 of 16)

[ oh ]
/ oʊ /


(used before a name in direct address, especially in solemn or poetic language, to lend earnestness to an appeal): Hear, O Israel!
(used as an expression of surprise, pain, annoyance, longing, gladness, etc.)

noun, plural O's.

the exclamation “O.”

Origin of O

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


O oh owe

Definition for o (3 of 16)


Grammar. object.

Definition for o (4 of 16)



the fifteenth in order or in a series.
the Arabic cipher; zero.
(sometimes lowercase) the medieval Roman numeral for 11.Compare Roman numerals.
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.
Chemistry. oxygen.

Definition for o (5 of 16)

[ uh, oh ]
/ ə, oʊ /


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

Origin of o'

Middle English; by shortening.

Definition for o (6 of 16)


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

Origin of O'

representing Irish ó descendant, Old Irish au

Definition for o (7 of 16)



an abridgment of ortho-.

Definition for o (8 of 16)


variant of ob- before m: omission.

Definition for o (9 of 16)


variant of oo-: oidium.

Definition for o (10 of 16)

Origin of o.

From the Latin word octārius

Definition for o (11 of 16)


Baseball. out; outs.

Definition for o (12 of 16)


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).
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

Definition for o (13 of 16)


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

Definition for o (14 of 16)


(in prescriptions) a pint.

Origin of O.

From the Latin word octārius

Definition for o (15 of 16)

Definition for o (16 of 16)

[ om-i-kron, oh-mi- ]
/ ˈɒm ɪˌkrɒn, ˈoʊ mɪ- /


the 15th letter of the Greek alphabet (O, o).
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. 2020

Example sentences from the Web for o

British Dictionary definitions for o (1 of 8)



/ (əʊ) /

noun plural o's, O's or Os

the 15th letter and fourth vowel of the modern English alphabet
any of several speech sounds represented by this letter, in English as in code, pot, cow, move, or form
another name for nought

British Dictionary definitions for o (2 of 8)


symbol for

chem oxygen
a human blood type of the ABO groupSee universal donor
logic a particular negative categorial proposition, such as some men are not married: often symbolized as SoPCompare A, E, I 2

abbreviation for

Australian slang offence

Word Origin for O

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

British Dictionary definitions for o (3 of 8)

/ (əʊ) /


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

British Dictionary definitions for o (4 of 8)



short for ortho- (def. 4)

British Dictionary definitions for o (5 of 8)

/ (ə) /


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

British Dictionary definitions for o (6 of 8)



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

Word Origin for -o

probably special use of oh

British Dictionary definitions for o (7 of 8)

/ (əʊˈmaɪkrɒn, ˈɒmɪkrɒn) /


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

Word Origin for omicron

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

British Dictionary definitions for o (8 of 8)


connective vowel

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

Word Origin for -o-

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

Medical definitions for o (1 of 3)



ortho- (often italic)

Medical definitions for o (2 of 3)


Used as a connective to join word elements:acidophilic.

Medical definitions for o (3 of 3)

[ ŏmĭ-krŏn′, ōmĭ- ]


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.

Scientific definitions for o (1 of 2)


The symbol for oxygen.

Scientific definitions for o (2 of 2)

[ ŏksĭ-jən ]


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.