Meaning of “o'clock” - Learner’s Dictionary

o'clock

adverb us uk /əˈklɒk/
Extra Examples
I've set the alarm clock for six o'clock.The meeting is scheduled to begin at two o'clock.The plane does not arrive until eleven o'clock.The deadline for applications is at three o'clock on Friday.He didn't leave work until nine o'clock.
one/two/three, etc o'clock

A1 used after the numbers one to twelve to mean exactly that hour when you tell the time:

It was ten o'clock when we got home.

(Definition of “o'clock” from the Cambridge Learner’s Dictionary © Cambridge University Press)