O'clock does not really have any distinct slang meaning, beyond the literal meaning.
O'clock is simply used to specify the time of day or night and often used in a casual or informal way and sometimes it's used humorously to refer to a time in which a particular activity is done.
Some examples of using O'clock humorously include.
It's chocolate o'clock for the kids.
It's wine "O'clock.
And if somebody says that someone or something is at 12 o'clock that means it's straight ahead of them.
O'clock is fundamentally used to indicate the hour on a clock, such as "it's 5 O'clock.
And in some contexts, such as in military or aviation, the O'clock can refer to direction, with the 12 O'clock being directly ahead.
The term O'clock is a contraction of "of the clock," which was used historically to specify the time according to the clock.