noun (1), often capitalized, often attributive
\ ˈō How to pronounce o (audio) \
plural o's or os\ ˈōz How to pronounce o (audio) \

Definition of o

 (Entry 1 of 11)

1a : the 15th letter of the English alphabet
b : a graphic representation of this letter
c : a speech counterpart of orthographic o
2 : a graphic device for reproducing the letter o
3 : one designated o especially as the 15th in order or class
4 : something shaped like the letter O especially : zero

Definition of o (Entry 2 of 11)

1 ocean
2 ohm
3 old
4 order
5 over

less common spelling of oh

1 used to express an emotion (such as surprise or desire) or in response to physical stimuli Oh, no! I forgot my key.Oh, don't stop!
2 used in direct address oh, waiter! Will you come here, please?
3 used to express acknowledgment or understanding of a statement Oh, yes. It's one of my favorite cities.
4 used to introduce an example or approximation Their house is about, oh, I'd say four miles from here.
\ ˈō How to pronounce O (audio) \

Definition of O (Entry 4 of 11)

: the one of the four ABO blood groups characterized by the absence of antigens designated by the letters A and B and by the presence of antibodies against these antigens

Definition of O (Entry 5 of 11)

1 offense
2 Ohio
3 [New Latin octarius] pint



Definition of O (Entry 6 of 11)

variants: or oo-

Definition of o- (Entry 7 of 11)

: egg oology specifically : ovum oogonium

Definition of o- (Entry 8 of 11)

orth- or ortho-



Definition of -o- (Entry 9 of 11)

used as a connective vowel originally to join word elements of Greek origin and now also to join word elements of Latin or other origin speedometerelastomer

Definition of -o (Entry 10 of 11)

: one that is, has the qualities of, or is associated with bucko


interjection suffix

Definition of -o (Entry 11 of 11)

in interjections formed from other parts of speech cheeriorighto

Synonyms for o

Synonyms: Noun (1)

Visit the Thesaurus for More 

Examples of o in a Sentence

Noun (1) a word that starts with an o a word that starts with o

First Known Use of o

Noun (1)

before the 12th century, in the meaning defined at sense 1a

Noun (2)

1929, in the meaning defined above

History and Etymology for o

Combining form

Greek ōi-, ōio-, from ōion — more at egg entry 1


Greek, thematic vowel of many nouns and adjectives in combination

Noun suffix

perhaps from oh entry 1

Interjection suffix

probably from oh entry 1

Buying Guide

Check out the 11 best games for word lovers from our Reviews team.

Learn More About o

Time Traveler for o

Time Traveler

The first known use of o was before the 12th century

See more words from the same century

Dictionary Entries Near o




See More Nearby Entries 

Statistics for o

Cite this Entry

“O.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/o. Accessed 13 Aug. 2022.

Style: MLA
MLACheck Mark Icon ChicagoCheck Mark Icon APACheck Mark Icon Merriam-WebsterCheck Mark Icon

More Definitions for o


noun, often capitalized
\ ˈō \
plural o's or os\ ˈōz \

Kids Definition of o

1 : the 15th letter of the English alphabet


Kids Definition of O

variant of oh



Kids Definition of O.




Medical Definition of o-

 (Entry 1 of 4)

orth-; ortho-


\ ˈō How to pronounce O (audio) \

Medical Definition of O (Entry 2 of 4)

: the one of the four ABO blood groups characterized by the absence of antigens designated by the letters A and B and by the presence of antibodies against these antigens



Medical Definition of O (Entry 3 of 4)

1 opening
2 [Latin octarius] pint used in writing prescriptions



Medical Definition of O (Entry 4 of 4)


More from Merriam-Webster on o

Nglish: Translation of o for Spanish Speakers


Test Your Vocabulary

Commonly Confused Words Quiz

  • vector image of a face with thought expression
  • I went to the ______ store to buy a birthday card.
Name That Thing

Test your visual vocabulary with our 10-question challenge!

Universal Daily Crossword

A daily challenge for crossword fanatics.

Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!