This Is What the “O” in “O’clock” Stands For

You probably say it every day—"It’s 5 o’clock," "Let’s meet at 3 o’clock"—without ever stopping to ask:What does the “o” in “o’clock” actually mean?

It turns out, this tiny letter holds a piece of linguistic history that dates back centuries. And once you know its origin, you’ll never hear the phrase the same way again.

The “O” Is Short for Something

The “o” in “o’clock” is actually a contraction of the word “of.”So when you say “6 o’clock,” what you’re really saying is:“6 of the clock.”

That may sound odd today, but in earlier centuries—especially around the 15th and 16th centuries—it was common to specify that a time referred specifically to the clock time, not the sun, church bells, or other natural time markers.

Why Say “of the Clock”?

Back in the day, people told time using a variety of methods: sundials, hourglasses, candles, and even bells rung at certain intervals. But with the rise of mechanical clocks in Europe, it became important to distinguish “clock time” from “natural time.”

So someone might say:

“It is five of the clock,”to mean:“It is five, as measured by the mechanical clock.”

Over time, this phrase was shortened in everyday speech to “five o’clock.” And the abbreviation stuck.

A Living Fossil in Language

Much like other contractions—such as “won’t” (will not) or “can’t” (cannot)—“o’clock” is a remnant of how English speakers used to talk. It’s a linguistic fossil that has endured simply because it became so familiar, even after the original meaning faded from everyday use.

Why We Don’t Say “of the Clock” Anymore

Language naturally evolves for simplicity. As mechanical clocks became the norm, there was no longer a need to specify how time was being measured. But the phrase had already taken root, and “o’clock” became the standard way to express time on the hour.

You’ll still hear it today in casual speech, storytelling, and formal announcements—though in writing, people often just say “at 5 PM.”

Final Tick

So the next time someone says “It’s 9 o’clock,” you’ll know that they’re echoing a phrase that’s over 500 years old—one that marks a moment in history when timekeeping was becoming standardized and the ticking of the clock was beginning to shape daily life.

Want more bite-sized language facts? Explore MindMatter’s Language & Origins section and discover the surprising stories behind the words you use every day.