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• O
  O

  noun

  the fifteenth letter of the English alphabet, a vowel.
• o'
  o'

  preposition

  a shortened form of of, as in o'clock or will-o'-the-wisp.
• O'
  O'

  a prefix meaning “descendant,” in Irish family names.
• o-
  o-

  an abridgment of ortho-.
• -o
  -o

  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 ).
• o.
  o.

  abbreviation

  pint.
• -o-
  -o-

  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.
• O.
  O.

  abbreviation

  (in prescriptions) a pint.
• o
  o

  noun

  the 15th letter and fourth vowel of the modern English alphabet
• O'-
  O'-

  prefix

  (in surnames of Irish Gaelic origin) descendant of

O

¹ American  

[oh] / oʊ /

Or o

noun

O's, plural Os, plural o's, plural os, plural oes plural

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.

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O ² American  

[oh] / oʊ /

interjection

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.)

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noun

O's plural

1. the exclamation “O.”

O ³ American  

abbreviation

1. Grammar. object.

2. Old.

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O ⁴ American  

Symbol.

1. the fifteenth in order or in a series.

2. the Arabic cipher; zero.

3. (sometimes lowercase) the medieval Roman numeral for 11.

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.

5. Chemistry. oxygen.

6. Logic. particular negative.

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o' ⁵ American  

[uh, oh] / ə, oʊ /

preposition

1. a shortened form of of, as in o'clock or will-o'-the-wisp.

2. Chiefly Dialect. a shortened form of on.

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O' ⁶ American  

1. a prefix meaning “descendant,” in Irish family names.

   O'Brien; O'Connor.

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o- ⁷ American  

Chemistry.

1. an abridgment of ortho-.

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o- ⁸ American  

1. variant of ob- before m:

   omission.

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o- ⁹ American  

1. variant of oo-.

   oidium.

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-o ¹⁰ American  

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.

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o. ¹¹ American  

abbreviation

1. pint.

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o. ¹² American  

abbreviation

1. octavo.

2. off.

3. old.

4. only.

5. order.

6. Baseball. out; outs.

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-o- ¹³ American  

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.

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O. ¹⁴ American  

abbreviation

1. (in prescriptions) a pint.

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O. ¹⁵ American  

abbreviation

1. Ocean.

2. octavo.

3. October.

4. Ohio.

5. Old.

6. Ontario.

7. Oregon.

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O ¹ British  

symbol

1. chem oxygen

2. a human blood type of the ABO group See universal donor

3. logic a particular negative categorial proposition, such as some men are not married: often symbolized as SoP Compare A E I 2

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abbreviation

1. slang offence

o ² British  

/ əʊ /

noun

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

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O ³ British  

/ əʊ /

interjection

1. a variant spelling of oh

2. an exclamation introducing an invocation, entreaty, wish, etc

   O God!

   O for the wings of a dove!

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O'- ⁴ British  

prefix

1. (in surnames of Irish Gaelic origin) descendant of

   O'Corrigan

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-o ⁵ British  

suffix

1. forming informal and slang variants and abbreviations, esp of nouns

   wino

   lie doggo

   Jacko

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-o- ⁶ British  

1. used to connect elements in a compound word Compare -i-

   chromosome

   filmography

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o- ⁷ British  

prefix

1. short for ortho-

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o' ⁸ British  

/ ə /

preposition

1. informal shortened form of of

   a cup o' tea

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Other Word Forms

Noun Inflected Forms

• O's
  plural
• Os
  plural
• o's
  plural
• oes
  plural
• os
  plural

Etymology

Origin of O²

First recorded in 1125–75; Middle English, from Old French, from Latin ō

Origin of o'⁵

From Middle English; by shortening

Origin of O'⁶

Representing Irish ó descendant, Old Irish au

Origin of -o¹⁰

Perhaps originally the interjection O, appended to words as in -o 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

Origin of o.¹¹

From the Latin word octārius

Origin of -o-¹³

Middle English (< Old French ) < Latin < Greek

Origin of O.¹⁴

From the Latin word octārius

Example Sentences

“Coffee badging”—going to the office just long enough to show your face and grab a cup o’ joe—was a popular workaround.

From The Wall Street Journal • Dec. 30, 2025

I was raised about two blocks from where you lived at the time, near Tail o’ the Pup.

From Los Angeles Times • Jan. 22, 2025

Any way you slice it, this week’s Slate News Quiz is a piece o’ fun.

From Slate • Nov. 1, 2024

"She worked so hard, to get up at silly o' clock... and to wear corsets for hours on end," she said, adding that she continued working right up to the end of her life.

From BBC • Sep. 28, 2024

What the books said was, as in the case of East o’ the Sun, interesting, but the idea that I could enter this world at any time I chose was even more attractive.

From "Bad Boy" by Walter Dean Myers