Longman Dictionary of Contemporary English - o'clock

~ adv 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  ("What time is it?" "It's 9 o'clock.".)  ( USAGE NOTE: O'CLOCK UK-US DIFFERENCE O'clock is used only when you are talking about the exact hour nine o'clock. Compare ten to nine Minutes after the hour are expressed with past (usually after in American English) five/a quarter/ten past nine Compare five/ten/a quarter after nine (American English only). In British English you can say `half past' the hour, but it is not possible in American English to say `half after' the hour. You talk about minutes before the hour using to (and also of in American English) e.g. twenty/a quarter to eight. Compare It's five minutes of two/a quarter of eight (American English only). Time can also be expressed by using the numbers alone The meeting is at 10.15. | I'll pick you up about 3.20. (=said three-twenty). )