What does the "o" stand for in 9 o'clock?

Answer 1

of the clock

Answer 2

As has been noted, "o'clock" is short for "of the clock". The fuller form of the expression is found as early as the late 14th century (in Chaucer).
http://www.randomhouse.com/wotd/index.pperl?date=19970102

But why "OF the clock"?

Some will note that the basic sense is "according to the clock". But I'm not sure that helps too much. So let's go a bit further, to the origins of the mechanical clock itself and the very word "clock".

"Clock" comes from Latin 'cloca' meaning "BELL". It was the term applied to the newer devices in use for measuring time in the thirteenth century. As the word itself suggests, the key feature of these clocks is the bell that chimed the hours In fact, these clocks, often set out in public in a tower to make it possible to notify everyone, did not necessarily have hands.

The key thing, then, was notifying people of the HOUR of the day (NOT the minute, and not, at least at first, the quarter or half hour either). This practice of marking/announcing the hours is echoed by the watchman's call "nine o'clock and all is well." Thus it is scarcely surprising that this way of expressing the time did not (and still does not, except in certain highly formal uses) state anything BUT the HOUR.
http://en.wikipedia.org/wiki/Clock#A_new_mechanism

So you may picture the situation around 1300 -- the bell in the 'clock tower' strikes ONE time announcing that the hour is "one of ['according to'] the clock".