From Longman Dictionary of Contemporary Englisho'clocko'clock /əˈklɒk $ əˈklɑːk/ ●●● S1 W3 adverb → one o'clock/two o'clock etc
Examples from the Corpus
o'clock• It's 5 o'clock.• It's exactly 10 o'clock.Origin o'clock (1400-1500) of the clock 