This is actually a phonological question.
Merriam Webster's etymology entry for o'clock explains that it is a
contraction of of the clock
But, 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?
I'd say the process of contracting more than one word (such as of the) to o' is somewhat productive, but the likelihood of any of the modern products doing more than reflecting "swallowed words" in dialogue is low since it reflects somewhat lower-class or uneducated speech.
That is, except for in the few cases where for historical reasons the contraction has become the standard, for example where Jack-o'-lantern came from Jack with the lanthorn (Etymonline).
But consider the following from Slammin' tar: a novel a 1998 novel by Cecil Foster,
'Cause all I would say is that the next-door neighbour had was to
phone for the police. And I'm in my little house when all o'
sudden I hear one loud knockin' on the door 'bout midnight and I
wondering who in the village know I'm home and coming already to try
to borrow a piece o' change from me.
where it seems that of a has been contracted to o' to reflect the casual speech of the character here. It's hard to see something like this becoming standard or being written in serious prose.
Here an example of o'hill as a contraction for of the hill in John Masefield's play Campden Wonder scene 2 (produced in London 1907)
PARSON
What do you know of this hair-string, John Perry? Do you know it?
JOHN
I know it, I do. And Mother know it, and Dick. To our cost we know it.
It be the cord we murdered en with.
JOAN AND DICK
Oh, John Perry!
PARSON
Where was this? This that you say?
JOHN
Below Battle Ridge. At the foot o’hill it were. By the brook,
where they found ens collar.
JOAN
Us was never nigh the place.
The other possible time where this might come up is in places where space is at a premium. In the Radio annual and television year book (1961) there's an entry for a radio show by the name of King O' Hill, and it's clear that it's not the presenter's name either since his name is printed separately.
So, while the contraction is far from standard or even acceptable in most instances, it's not entirely restricted to fixed phrases either.
The rather more common contraction of to o' with no other words like the, a , etc. also subsumed by the contraction is still mostly reflective of uneducated, casual or colloquial speech in the modern day rather than a lexicalised form that is used in prose proper: pot o’gold, piece o’ cake, cup o’ joe, sweet child o’mine, time o’day, cup o’tea, etc. Except again in cases where it's become the standard for historical reasons like man o’war, or Land O’Lakes.
The OED has the following on the Etymology page for preposition of
The variant o became rare after 1400. But in the late 16th and the
17th, and 18th centuries the preposition was often used colloquially,
and in the dramatists, reduced to o' (e.g. (especially) o' the ‘on
the’ (compare i' th': see in prep.), also o' my life (Shakespeare), o'
my conscience (Sheridan), etc.), a form still found in Scottish and
northern English regional usage. In o' nights and similar expressions
it is often interpreted as meaning ‘of’ (compare the phrase of a
night).
