O’clock is a contraction of “of the clock†or “on the clock†that means “according to the clockâ€: ‘We are expected to be there at seven o’ clock in the morning.’
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.’
Use EITHER o’clock OR a.m./p.m. in a sentence (NOT both). Compare: ‘The work should be completed by seven o’clock.’ ‘The work should be completed by seven p.m.’
Do not use o’clock after 6.00, 7.00 etc. Compare: ‘8 a.m.’, ‘8.00’, ‘8.00 a.m.’, ‘8 o’clock’.