Oxford Advanced American Dictionary

Definition of o'clock adverb from the Oxford Advanced Learner's Dictionary

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o'clock

adverb
o'clock pronunciation English əˈklɒk o'clock pronunciation American əˈklɑːk
 
used with the numbers 1 to 12 when telling the time, to mean an exact hourHe left between five and six o'clock.at/after/before eleven o'clock