What is the correct contraction for clock?

The correct contraction for "of the clock" is o'clock. Coined around 1720 CE, the term derives directly from the Middle English term that is attested from 1389 CE.

Contractions are defined as the shortened forms of Words or groups of Words, with the omitted letters often replaced by an apostrophe, i.e; isn't (is not), haven't (have not) and won't (will not). They occur primarily, but not exclusively, in informal writings and speeches, but are rare in scientific and scholarly writings.