text: O is a word in English, used as an interjection to indicate the vocative case, as in "O ye of little faith!" So, when it's used like that...as in O Lord or O Wise One, it means you are calling out to someone. \n\nO'clock is a different case. Here, it means of (or according to) the clock. This is just a shortened form of the original phrasing...like it's 5 of the clock.