used after a number from one to twelve to say the time when it is exactly that hour.
It's two o'clock.
He called me at four o'clock in the morning.
The meeting is scheduled for 3 o'clock in the afternoon.
We'll meet at the park entrance at 9 o'clock sharp.
The train departs at 7 o'clock, so be sure to arrive early.
The store closes at 5 o'clock, so plan your shopping accordingly.
