[adverb] [not gradable] - used after a number from one to twelve to say the time when it is exactly that hour"Have you got the time?" "It's two o'clock."I got a phone-call from him at four o'clock in the morning.It's almost ten o'clock, the news will be on soon.
O'CLOCK
Meaning of O'CLOCK in English
Cambridge English vocab. Кембриджский английский словарь. 2012