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

o'clock

adverb uk us /əˈklɒk/
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)