Hi, I found this website out of curiosity (I'm an English-Spanish translator & linguist, lover of all things related to languages) and so far I'm liking it! =)

Anyway, this is an interesting question, so I looked it up in the Online Etymology Dictionary website (http://www.etymonline.com), from which I quote:

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o'clock
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.).

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Keep up the good work with your website! :D