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

1 used to express an emotion (such as surprise or desire) or in response to physical stimuliOh, no! I forgot my key.Oh, don't stop!
2 used in direct addressoh, waiter! Will you come here, please?
3 used to express acknowledgment or understanding of a statementOh, yes. It's one of my favorite cities.
4 used to introduce an example or approximationTheir 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 originspeedometerelastomer

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 speechcheeriorighto

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Synonyms for o

Synonyms: Noun (1)

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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

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Learn More about o

Time Traveler for o

Time Traveler

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

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Dictionary Entries near o








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Statistics for o

Cite this Entry

“O.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/o. Accessed 24 Sep. 2020.

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More Definitions for o

How to pronounce -o (audio)

English Language Learners Definition of o

: the 15th letter of the English alphabet
: the number zero
used as a mark at the end of a letter, an e-mail, etc., to represent a hug


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)


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