While the analog clock has fallen out of fashion, it does reveal what the "o" in "o'clock" means.
“O’clock” is a shortened form of the phrases “of the clock” and “of clock,” which people began using to communicate the time ...
The traditional time-of-day clock that displays 12-hour marks around a circle with midnight and noon at the top. The analog clock's minute hand goes around the circle every hour, and the shorter hour ...