o3
O, oxygen +
O Canada, the national anthem of Canada +
O Little Town of Bethlehem, a Christmas song +
O. Robertson and the blood bank, 1917 +
oat, a cereal plant grown in cool climates +
oatmeal, for people +
oats, for horses +
obsessive-compulsive disorder, OCD -
obey the call of nature +
obey the Commandments +
obey the commands +
objective account of the problem, an +
objects of prehistoric date +
oblique angle +
oblivious of her surroundings -
oblivious to her surroundings -
obstacle course +
obstacle race +
obstruct the view -
obstructing the police -
obtuse angle, an angle of more than 90° and less than 180° +
occasional showers -
occasional table +
occipital lobe and frontal lobe
octopus, a sea mollusk with eight long tentacles +
octopus and the ink screen +
occult blood -
occupational hazard -
occupational therapy +
occupy the position of manager +
OCD, caused by failure of control over one's thoughts and actions -
OCD, characterized by performing certain routines repeatedly -
OCD, obsessive-compulsive disorder -
o'clock, of the clock +
octane number, a measure of the anti-knock properties of gasoline +
Odell and Matchbox, 1953 +
odor bomb, a defense +
odds and ends -
Oedipus Complex, sexual feelings toward one's parents -
of age +
of considerable importance +
of course +
of French descent +
of happy memory +
of less importance -
of no avail -
of no consequence -
of no significance -
of noble birth +
of slender build +
of the first magnitude +
of unsound mind -
off and on +
off balance -
off chance -
off color -
off course -
off duty -
off guard -
off hour -
off limits -
off the mark -
off the record -
off the track -
off year -
offend against the law -
offensive remarks -
offensive smell, an -
offensive weapons -
offer the prospect of promotion +
office boy, an +
office hours, nine to five +
office politics -
office seeker +
office worker +
officer of the day +
officer of the guard +
official red tape -
offset printing; printing from a plate to a rubber blanket, then to the printing surface +
often mistaken for her sister -
Oh God! -
Ohio State University and the solar battery, 2014 +
Ohno and junk DNA, 1972 +
oil base, an +
oil concession, an +
oil field +
oil paint +
oil painting +
oil rig, an +
oil royalties +
oil shale +
oil tanker +
oil the wheels +
oil well +
oiled silk +
old age -
old boy +
old country +
old girl +
Old Glory -
old goat -
old hand +
old hand at negotiating, an +
old maid -
Old Nick, the Devil -
old people's home, an +
old school -
Old Testament, written in Hebrew, c. 450 BC +
Old Testament and the New Testament, the +
Old Testament and the New Testament, Bible, the +
old witch -
old wives' tale -
Old World -
old year -
older and wiser +
olive branch +
olive oil +
Olympic motto: "Faster, Higher, Stronger", the +
Olympic torch relay, the, 1936 +
omega, the last letter of the Greek alphabet +
on average +
on a grand scale +
on a knife edge -
on a shoestring -
on all fours -
on alternate days +
on American soil +
on an even keel +
on and off +
on balance +
on board +
on call +
on cloud nine (informal) +
on duty +
on earth +
on fire -
on first sight +
on foot +
on friendly terms +
on hand +
on holiday +
on high +
on horseback +
on leave +
on location +
on my doorstep +
on my guard +
on my hands +
on my mind +
on my own initiative +
on my own time -
on my part +
on no consideration -
on oath +
on occasion +
on paper +
on parade +
on patrol +
on pins and needles -
on principle +
on probation +
on purpose +
on receipt of your letter +
on record +
on remand -
on request +
on sale +
on schedule +
on short notice -
on strike +
on Sunday +
on Sunday, Oct. 16, 2016 +
on the air +
on the back burner +
on the contrary -
on the defensive +
on the downgrade -
on the eve of an election +
on the first floor +
on the fly +
on the go +
on the highway +
on the hoof +
on the horizon +
on the house +
on the ice -
on the inside +
on the Internet +
on the loose +
on the march +
on the mark +
on the market +
on the mend +
on the move +
on the next day +
on the other hand +
on the phone +
on the plea of ill health +
on the premises +
on the rebound +
on the right track +
on the road +
on the run -
on the safe side +
on the scene +
on the sea +
on the shelf -
on the sick list +
on the sidelines -
on the sly -
on the spot +
on the tip of my tongue -
on the upgrade +
on the wane -
on the warpath -
on the watch +
on the whole +
on the wing +
on the wrong side of the tracks -
on the wrong side of seventy, over seventy years old -
on the wrong tack -
on the wrong track -
on this assumption +
on time +
on tiptoe -
on top +
on top of the world +
on tour +
on trial +/-
on trust -
once and for all +
once in a blue moon -
once in a while +
once more +
once upon a time +
once a thief, always a thief -
one and two and three and four and fifteen +
one and two and three and four and five +
one and two and three and four and ten +
"one country, one system" +
"one country, two systems", a constitutional principle +
one day +
one husband, one wife +
one husband, four wives -
one husband, seven wives -
one jump ahead +
one more, two more, three more +/-
one one-thousand +
one-child policy +/-
one-eye -
one-hand -
one-head +
one-horn +
one-hour delay, an -
one-leg -
one-man job, a +
one-night stand, a -
one-off -
one-track mind -
one-up +
one-way street +
one-way ticket +
one-way traffic +
one-wheel +
one-wing -
onion soup +
only a matter of time +
only a slim chance of success -
Onnes and superconductivity, 1911 +
onset of winter, the +
open a business +
open a debate +
open and close +
open championship, an +
open classroom +
open fire -
open house +
open-heart surgery +
open letter +
open mind +
"Open Sesame!", a command and password +
open or close +/-
open question +
open secret -
open the bottle +
open the box +
open the door +
open the gift +
open the window +
open to question -
opera house, the +
operating in relays +
operating room +
ophthalmoscope, an instrument for examining the retina of the eye +
opium, an analgesic +
opium tincture, an antidiarrheal +
Oppenheimer and the atom bomb, 1945 +
opportunity knocks +
opposite sex, the +
opt in +
opt out -
optic nerve +
optical art +
optical illusion +
optical scanner +
or so +
oral evidence +
oral history +
oral polio vaccine +
orange for general surgery +
orange stick +
orchestra pit +
order book +
order form +
order of God's Creation +
order of ham and eggs, an +
orderly mind, an +
ordinary decent people +
Oresme and the plus sign, 1360 +
organ transplant +
organic chemistry, the chemistry of C and H, N, O, P or S +
organic food, produced without the use of artificial fertilizers and pesticides +
organic matter +
organize a picnic +
organized crime -
Orient and salaam, the +
orient myself +
orient the new students +
Oriental rug +
origami, the art of paper folding +
original mind, an +
Oscars, awards for outstanding achievements in film industry +
O'Shaughnessy and marijuana, 1839 +
ostrich, a large, swift-running bird that cannot fly +
ostrich, said to bury its head in the sand when pursued hard -
other day, the +
other times, other manners -
other week, the +
other world +
Otis and the elevator, 1861 +
ought to be here by now +
Oughtred and the multiplication sign, 1631 +
Oughtred and the slide rule, 1621 +
our last hope +
our little boy +
out after dark -
out box +
out of alignment -
out of bounds +
out of breath -
out on bail +
out of character -
out of commission -
out of context -
out of control -
out of date -
out of debt +
out of doors +
out of fashion -
out of favor -
out of focus -
out of gear -
out of hand -
out of harness -
out of his own mouth +
out of joint -
out of line -
out of luck -
out of my hands -
out of my mind -
out of order -
out of place -
out of pocket -
out of position -
out of practice -
out of print -
out of season -
out of sequence -
out of service -
out of sight -
out of sight, out of mind -
out of step -
out of stock -
out of the frying pan and into the fire -
out of the picture -
out of the way +
out of the woods +
out of this world +
out of touch -
out of true -
out of tune -
out of turn -
out of work -
out of style, out of fashion -
out on a limb -
outboard motor +
outdoor activities +
outdoor games +
outer ear +
outer island, an -
outer space +
outreach program +
Oval Office, the; the official office of the President of the US +
over and above +
over and over -
over the long haul +
over the phone +
over the years +
over to you +
overdo it -
overdo things -
overdressed for the party -
overran its allotted time -
overthrew the government +
Owen, Dubernard, and the hand transplant, 1998 +
owl flits, the (motion) +
owl hoots, the (sound) +
own a laundry business +
oxycodone, a moderately potent opioid analgesic +
OxyContin, a controlled release formulation of oxycodone +
oxygen, a life-supporting gas for animals +
oxygen and ozone +
ozone, a pale blue gas with a distinctive pungent smell +
ozone, produced from dioxygen by the action of UV light and by lightning bolts +
ozone hole, caused by the use of CFC -
ozone layer, a region of the Earth's stratosphere that absorbs the sun's UV radiation +