o'clock S1 W3 /əˈklɒk $ əˈklɑːk/ 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.

[TahlilGaran] Dictionary of Contemporary English ▲

o'clock
adverb
1.
BAD: It was twenty past four o'clock when the train arrived.
GOOD: It was twenty past four when the train arrived.
BAD: They finished their dinner at about 7.30 o'clock.
GOOD: They finished their dinner at about 7.30.

Usage Note:
Do not use o'clock for times that include minutes or parts of an hour. Compare: 'It's four o'clock.' 'It's ten past four.'

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2.
BAD: By seven o'clock p.m. the child had been found.
GOOD: By seven p.m. the child had been found.
GOOD: By seven o'clock (in the evening) the child had been found.

Usage Note:
Use EITHER o'clock OR a.m./p.m. (NOT both).

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3.
BAD: I start work at 9.00 o'clock.
GOOD: I start work at 9 o'clock.

Usage Note:
Do not use o'clock after 6.00, 7.00 etc. Compare: '8 a.m.', '8.00', '8.00 a.m.', '8 o'clock'.

[TahlilGaran] Dictionary of Common Errors ▲