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

Definition of

(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


abbreviation (1)

Definition of (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 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 \

Definition of (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


abbreviation (2)

Definition of (Entry 5 of 11)

1 offense

2 Ohio

3 [ New Latin octarius ] pint



Definition of (Entry 6 of 11)


variants: or oo-

Definition of o- (Entry 7 of 11)

: egg oology specifically : ovum oogonium


abbreviation (3)

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

Definition of -o (Entry 11 of 11)

in interjections formed from other parts of speech cheeriorighto

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Synonyms for o

Synonyms: Noun (1)

aught, cipher, goose egg, naught (also nought), nil, nothing, oh, zero, zilch, zip

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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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Dictionary Entries near o








Statistics for o

Look-up Popularity

Time Traveler for o

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

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More Definitions for o



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
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 \

Medical Definition of (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 (Entry 3 of 4)

1 opening

2 [ Latin octarius ] pint used in writing prescriptions



Medical Definition of (Entry 4 of 4)


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More from Merriam-Webster on o

Thesaurus: All synonyms and antonyms for o

Spanish Central: Translation of o

Nglish: Translation of o for Spanish Speakers

Comments on o

What made you want to look up o? Please tell us where you read or heard it (including the quote, if possible).


a soft lustrous wool fabric with mohair

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