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Me and a couple of friends were just trying to figure out where the *** the 'o' in o'clock comes from, and what the point with saying it is. Is it a preposition? Like "on" or "of" without the last letter?
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Yes. When watches were a novelty, folks uses to exclaim, 'My goodness! It's almost twelve of the clock-- I'm going to miss my carriage!'

The o' is a leftover. Notice that we only use it on the hour. We don't say 'look-- it's twelve-fifteen o'clock!' Perhaps we retain the 'o'clock' just for the rhythmic balance of the length of phrase when we mention the time.
Comments  
Students: Are you brave enough to let our tutors analyse your pronunciation?
its not '0' its ZERO '0'
and this is not true this is a lie
Authoritative references please, Mr. Phonixor.