o

The definition of o is the fifteenth letter of the English alphabet.

(noun)

An example of o is the first letter in the name "Obama."

See o in Webster's New World College Dictionary

or O

noun pl. O's, o's

the fifteenth letter of the English alphabet: from the Greek omega and omicron, both borrowed from the Phoenician
any of the speech sounds that this letter represents, as, in English, the vowel (ō) of boat, (ä) of hot, or (ô) of wrong
a type or impression for o or O
the fifteenth in a sequence or group
an object shaped like

adjective

noun pl. O's

popularly the figure zero; nought: in printed matter, zero is usually differentiated from the letter O, as in this dictionary: “O” is the character used for the letter, “0” is the character used for the number
a blood type

interjection

noun pl. O's

an instance of this exclamation

oxygen

of: will-o'-the-wisp

ortho- (sense ): usually italicized and hyphenated in chemical names

forming slangy, intensive counterparts of adjectives, nouns, and interjections: cheapo, freako
forming slang nouns from adjectives: weirdo, sicko

Origin: clipped < words ending in -o (e.g. hippo, steno, gringo)

forming compound words: a connective vowel originally used for combining Greek elements only, but now used freely: sadomasochism, meritocracy

descendant of: used in Irish surnames: O'Reilly

Origin: Ir ō, descendant

See o in American Heritage Dictionary 4

or O

noun pl. O's o's or O's also os or Os

The 15th letter of the modern English alphabet.
Any of the speech sounds represented by the letter o.
The 15th in a series.
Something shaped like the letter O.
O One of the four major blood groups in the ABO system. Individuals with this blood group have neither A nor B antigens on the surface of their red blood cells, and have both anti-A and anti-B antibodies in their blood serum.
A zero.

interjection

Used before the name of or a pronoun referring to a person or thing being formally addressed: “How can I put it to you, O you who prepare to travel with important matters on your mind?” (Jo Durden-Smith).
Used to express surprise or strong emotion: “O how I laugh when I think of my vague indefinite riches” (Henry David Thoreau).

The symbol for the element oxygen.

abbreviation