In the abbreviation o'clock what does the o stand for?

Answer 1

Much as don't is the abbreviation for do not.

The abbreviation o'clock is the short for "of the clock".

So you could say: "It is now 12 of the clock" instead of: "It is now 12 o'clock"

Answer 2

In the abbreviation "o'clock," the "o" stands for "of." It is a contraction of the phrase "of the clock."