Why do we say O'Clock ??

 O'Clock means of the clock.

300 - 400 years ago, people used 2 types of time.

1.  Sun time  --  By position of Sun  (  like on midday.  )

2.  Clock time  ---  By mechanical clock

To be clear which type of time they are referring to, people started saying   ----

3 of the clock  --  meaning clock time, not Sun time !!!

Over the time, of the clock shortened to   O'Clock  .....

So, 3 O'Clock   =   3   of the clock  !!!!