Real Dictionary

-o in American English
forming slangy, intensive counterparts of adjectives, nouns, and interjections
suffix: 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

-o in British English
suffix: forming informal and slang variants and abbreviations, esp of nouns

-o- in American English
forming compound words
connective vowel: 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 (Fran

-o- in British English
connective vowel: used to connect elements in a compound word

O
number: O is used to mean zero, for example when you are telling someone a phone number, or mentioning a year such as 1908.
variable noun: O is the fifteenth letter of the English alphabet.

O in American English
noun: the fifteenth letter of the English alphabet, a vowel

O in American English 1
the fifteenth letter of the English alphabet: from the Greek omega and omicron, both borrowed from the Phoenician
the figure zero; nought
interjection: (used before a name in direct address, esp. in solemn or poetic language, to lend earnestness to an appeal)
noun: the exclamation “O”

O in American English 2
octavo
used in direct address
abbreviation: Old

O in American English 3
Ocean
symbol: the fifteenth in order or in a series

O in American English 4
oxygen

o in British English
noun: the 15th letter and fourth vowel of the modern English alphabet

O in British English 1
symbol for: oxygen

O in British English 2
exclamation

o- in American English
ortho- (sense 4) ortho- (sense 4b)

o- in American English 1
combining form

o- in American English 2
prefix

o- in American English 3
combining form

o- in British English
prefix

o. in American English
abbreviation: Ocean
abbreviation: pint

o'
preposition: O' is used in written English to represent the word 'of' pronounced in a particular way.

o' in American English
of1
prefix: a prefix meaning “descendant,” in Irish family names
preposition: an abbreviated form of of, as in o'clock or will-o'-the-wisp

o' in British English
preposition

O'- in American English
descendant of

O'- in British English
prefix: (in surnames of Irish Gaelic origin) descendant of