“For example, saying ‘it’s 3 o’clock’ originally meant ‘it is 3 of the clock.’ Over time, ‘of the’ was shortened to just ‘O,’ leading to the phrase we commonly use today. ...

For example, saying "it's 3 o'clock" originally meant "it is 3 of the clock." Over time, "of the" was shortened to just "O'," leading to the phrase we commonly use today.