What does the o for o'clock stands for as a contraction?

The Words Of the clock form the contraction o'clock.

Sentences:

The elderly man looked at his timepiece. "I will give the boy till 3 of the clock to return the silver he stole!"

"I heard a loud noise at 1 of the clock, but I believed it was the maid stoking the fireplace. Then, at 2 of the clock, I found her at the bottom of the stairs."