An apostrophe is used in o'clock because the word is a contraction of the phrase "of the clock." Just like other contractions, the apostrophe takes the place of missing words or letters from a longer word or phrase. ... Always use the shortened form, "o'clock."

How do you write o clock?

At the beginning of each hour, we use the term “o'clock”. For example, “It's one o'clock” (1:00). Or “It's four o'clock” (4:00).

Why do we say one o clock?

The practice of saying “o'clock” is simply a remnant of simpler times when clocks weren't very prevalent and people told time by a variety of means, depending on where they were and what references were available.