o'clock S1 W3 /əˈklɒk $ əˈklɑːk/ BrE AmE adverb
[ Date: 1400-1500 ; Origin: of the clock ]
one o'clock/two o'clock etc one of the times when the clock shows the exact hour as a number from 1 to 12:
It’s already 9 o'clock.
The meeting is at 10 o'clock.
► Do not use o'clock when mentioning minutes or parts of an hour : ten past nine (NOT ten past nine o'clock) | half past one (NOT half past one o'clock) . Do not use o'clock when writing the time in the form 1.00, 2.00 etc.