Why is it "seven o'clock" instead of "seven clock"? [closed]

Why does "seven o'clock" contain "o'" instead of simply being "seven clock"? What is the historical linguistic origin of this "o'".

(This is actually a phonological question. If we look at a structurally similar expression such as king of the Hill, the construction appears to be the same. Why, then, did it not contract into something like king o'hill? In other words, why is the reduction found in o'clock not a general or productive process? Is there something special about o'clock that allowed this particular contraction to become lexicalized while other seemingly parallel expressions did not?)

Abbreviating "of" and "of the" to "o'" is not uncommon. In addition to o'clock, we also have the Irish convention of surnames beginning with "O'"; this comes from the Gaelic word "O" that means "descendant of".

And there are phrases like "will-o'-the-wisp" and "top o' the mornin'". In England "cup of tea" is often reduced to as just "cuppa".