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.
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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.'
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.
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Use EITHER o'clock OR a.m./p.m. (NOT both).
3
BAD : I start work at 9.00 o'clock.
GOOD : I start work at 9 o'clock.
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Do not use o'clock after 6.00, 7.00 etc . Compare: '8 a.m.', '8.00', '8.00 a.m.', '8 o'clock'.