of, by, or according to the clock (used in specifying the hour of the day): It is now 4 o'clock.
according to a method for indicating relative position whereby a plane in space is considered to be numbered as a clock's face, with 12 o'clock considered as directly ahead in horizontal position or straight up in vertical position.

1710–20; see o', clock1

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World English Dictionary
o'clock (əˈklɒk)
1.  used after a number from one to twelve to indicate the hour of the day or night
2.  used after a number to indicate direction or position relative to the observer, twelve o'clock being directly ahead or overhead and other positions being obtained by comparisons with a clock face
[C18: abbreviation for of the clock]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

c.1720, abbreviation of of the clock, from M.E. of the clokke (1389). Attested from 1904 in ref. to direction (by shooters, fighter pilots, etc.).
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
My partner is an early riser, usually stands up at five o'clock in the morning.
At the three o'clock shift change, when the afternoon crew would have arrived
  during the shuttles' working days, no one comes.
Jumped on the train and was back home by nine o'clock.
There are spiky projections in the picture of the inner core on opposite ends
  at about one and seven o'clock.
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