The editors of the online etymology dictionary (oed) obviously consulted (as did I) with the editors of the Oxford English Dictionary (OED), to whom I must defer in matters of my tortured tongue. This from my 1955 ed of the OUniversalD:
" 'O'clock' is short for of the clock; other variants were of clock, a clock (see A prep.2)"
At p. 326 under "clock" sb. 2.