When to say "o'clock"?

When Should You Use O'Clock?

If you have ever wondered why we add a funny little word to the end of certain times, you are not alone. The phrase o'clock is a classic English time-teller, but it comes with a few simple rules that keep our schedules running smoothly.

First and foremost, o'clock always follows a numeral. You will hear people say it is three o'clock or eight o'clock to let others know exactly what hour of the day it currently is. It acts as a helpful grammatical anchor.

However, it is strictly reserved for exact hours. You would never say that it is two-fifteen o'clock or nine-thirty o'clock—while people would certainly understand what you mean, it sounds completely unnatural to a native speaker. For minutes and fractions of an hour, you simply state the numbers directly, like half past four or ten-forty, leaving o'clock out of the mix entirely.

Historically, the expression actually comes from an old phrase meaning of the clock, which was a clever way to distinguish time told by a mechanical timepiece from time told by a sundial. Today, it remains your best friend whenever you are talking about hitting the top of the hour right on the dot.