Douglas Harper's Etymology Dictionary
c.1720, abbreviation of of the clock (1640s), from Middle English of the clokke (late 14c.). Use of clock hand positions to describe vector directions or angles is from late 18c.
Wiktionary
adv. (label en modifying a numeral, one to twelve) In conjunction with a numeral, indicates the time within a twelve hour period (midnight to noon or noon to midnight), specifically the time when the hour hand of a clock points precisely to the symbol or marking corresponding to the designated numeral.
WordNet
adv. according to the clock; "it's three o'clock in Tokyo now"
Usage examples of "o'clock".
It was not even ten o'clock, and the main street of Colgate was streaming with traffic, car stereos thumping.
It was full summer, as it is everywhere in mid-August, but at Carlsbad the sun was so late getting up over the hills that as people went to their breakfasts at the cafes up the valley of the Tepl they found him looking very obliquely into it at eight o'clock in the morning.
On schedule at eleven o'clock, Burt Forsyth got out of a taxi on Fifth Avenue.
At five o'clock on Tuesday morning, a local IRS agent named Ramos presented himself at the back door of the restaurant to fill in for his cousin, the busboy, who'd purportedly taken ill.
They had driven a hundred and ninety miles from Bruges, more or less, in under three hours and they were welcomed by the carillons ringing out eleven o'clock.
Bruce, early estimates are that hundreds died here when at least seven objects struck the ground in and around Carlisle at a little after eleven o'clock this evening.
By the time I closed the concession and headed for the meadow at the back of the county fairgrounds, where the carnies had established their mobile community, it was a few minutes after one o'clock.
These were Madame Langlois, Madame Caron, Madame Dubreuil, Madame Tuvache, and regularly from two to five o'clock the excellent Madame Homais, who, for her part, had never believed any of the tittle-tattle about her neighbour.
At nine o'clock in the morning, at the moment when the Courbevoie garrison was descending upon Paris, the placards of the _coup d'etat_ being still fresh upon the walls, Louis Bonaparte had left the Elysee, had crossed the Place de la Concorde, the Garden of the Tuileries, and the railed courtyard of the Carrousel, and had been seen to go out, by the gate of the Rue de l'Echelle.
Every day, ever since he had taken possession of the house, he had supervised the milking in the cow barns to measure with his own hand the quantity of milk that the three presidential wagons would carry to the barracks in the city, in the kitchen he would have a mug of black coffee and some cassava without knowing too well the direction in which the whimsical winds of the new day would blow him, always attent on the gabbling of the servants, who were the people in the house who spoke the same language as he, whose serious blandishments he respected most, and whose hearts he best deciphered, and a short time before nine o'clock he would take a slow bath in water with boiled leaves in the granite cistern built in the shadow of the almond trees of his private courtyard, and only after eleven o'clock would he manage to overcome the drowsiness of dawn and confront the hazards of reality.
This took place around one o'clock, and only then did Sister John relax and turn her attention to her cellmates, remedying her previous oversight by announcing that she was Sister John of St.
This morning, round about six o'clock, some chaps came along to open up the place, and they saw an old brown canvas trunk lying in the roadway where the dead end opens off the main street.
A few minutes past ten o'clock that morning, a Federal Express deliveryman entered the chichi apartment building.
That morning, she had again seen the old lady walk past, and behind her, in ones and twos, came a straggle of what she had realised were churchgoers, bound for the eight o'clock service.
There were usually, by seven o'clock, whiskey-and-soda glasses and tea-cups on most of the furniture, and half-smoked cigarets on everything that would hold them, including the piano.