What phrase is "o'clock" contracting?

According to The Time-traveller's Guide to Medieval England 'of the clock' was used to describe time when it was being sliced in 24 equal parts (hours) of the day.

It was used to differentiate the practice, used equally as much, of using solar time, whereby the 7th hour would shift in actual time, however would always be 7/12ths of a solar day after sunrise, and the length of an hour would increase in summer and decrease in winter.

I believe it's "of the clock".

Yes, o’clock is a contraction of the phrase “of the clock”. Online Etymology Dictionary traces the phrase back to the 1640s; the contraction first appears around 1720.¹

Etymonline says it’s an abbreviation of of the clock, which makes sense phonetically—both the f in of and the th in the are reduced to null in some contexts, and it’s a very common phrase.

The earliest citation they give is 1720, which is fairly late; prior to that, the phrase was spelled out in full—even if it was pronounced the same.