Operate = (1) direct or control something: ‘Do you know how to operate this machinery?’ (2) perform surgery on (medicine): ‘Have you heard what happened to the last patient he operated on?’ (3) to perform a function or work: ‘ The motor operates smoothly.’ ‘The camera also operates underwater.’ (4) to be involved in military activities: ‘A militant group  is operating against the government.’

Operation = (1) the state of being in effect or being operative: ‘That law is no longer in operation.’ (2) a planned activity involving many people performing various actions: ‘They planned a rescue operation.’ (3) a medical procedure involving an incision with instruments: ‘My mother is having an operation tomorrow.’ ‘Mr. Barrett is going to have an operation on his back.’

Once = (1) one time only: ‘You have to take this medicine once a day.’ (2) whenever; as soon as: ‘Once it stops raining, we can go out.’ (3) at some indefinite time in the past: ‘She was a very popular actress once.’ (4) used in negative sentences and questions, and after ‘if’ to mean ‘ever’ or ‘at all’: ‘He didn’t once thank me.’ ‘If she once decides to do something, it becomes difficult to change her mind.’

Often = (1) many times; frequently or in great quantities:  ‘The trains are often late.’ ‘They often go out to dinner.’ (2) in many cases or instances: ‘People are often afraid of things they don’t understand.’

Every so often = sometimes; occasionally: ‘I meet him at the club every so often.’ ‘Every so often I heard a strange noise outside.’

As often as not = quite frequently; usually; in a way that is typical of somebody/something: ‘As often as not, he’s late for work.’

Offence= (1) a feeling of anger caused by being offended: ‘He took offence at my slightest criticism.’ (2) the team that has the ball (or puck) and is trying to score: ‘Our team has the best offence in the league.’ Commit an offence (NOT do): ‘He is accused of committing various minor offences.’

O’clock is a contraction of “of the clock” or “on the clock” that means “according to the clock”: ‘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’.