The contraction of the word “of” to o’ is considered highly informal, but the phrase “o’clock” is somehow different.
This week on That’s What They Say, host Rina Miller and University of Michigan English Professor Anne Curzan discuss how we talk about time.
The expression “o’clock” comes from “of clock” as in “according to the clock,” says Curzan.
It might seem like an antiquated phrase, but "o'clock" is still used quite a lot. But, there is something else on the rise and that is the use of a.m. and p.m.