WHY WE SAY "O CLOCK"

Before the invention of the clock, people told time a variety of means, depending on where they were and what references were available.

Generally, the Sun was used as a reference point, with solar time time being slightly different than clock time. Clocks divide time evenly, whereas, by solar time, hour lengths vary somewhat based on a variety of factors, like what season it is.

Thus, to differentiate the fact that one was referencing a clock's time, rather than something like a sun time, as early as the fourteenth century one would say something like, "It is seven of the clock,," which later got slurred down to "seven o' clock" sometime around the sixteenth or seventeenth centuries. In those centuries, it was also somewhat common to just omit the "o" altogether and just say something like "seven clock".

While today with people using clock nowadays to tell time, it isn't necessary in most cases to specify we are referencing time from clocks, but the practice of saying "o'clock" has stuck anyway.

Source: Today found out.