O'clock is the short version of according to the clock and is often said when the hour is full. This means if the time is exactly 12:00, 1:00, 5:00, etc then we say o'clock at the end of our sentence. For example, 12:00 = ⁠It's twelve o'clock. or 9:00 = ⁠It's nine o'clock.. Many English speakers, however, will leave o'clock out and would instead pronounce these examples as ⁠It's twelve. or ⁠It's nine..