Andrew Cote, Sciences BSC, University of British Columbia, Canada
Everyone at the beach would get sunburn. Ozone is molecular oxygen, and blocks the majority of UV light. Without it we are toast.
Every internal combustion engine would stall. This means that every airplane taking off from a runway would likely crash to the ground, while planes in flight could glide for some time.
All pieces of untreated metal would instantly spot weld to each other. This is one of the more interesting side effects.
The reason metals don't weld on contact is they are coated in a layer of oxidation. In vacuum conditions, metal welds without any intermediate liquid phase.
Everyone's inner ear would explode. As mentioned, we would lose about 21 per cent of the air pressure in an instant (from sea level to 2000m elevation), so expect some serious hearing loss.
Every building made out of concrete would turn to dust. Oxygen is an important binder in concrete structures (really, the CO2 is), and without it the compounds do not hold their rigidity.
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Geoffrey Widdinson, Process Engineer, Texas
Every car and plane would stall, but they'd coast for a few seconds then easily restart. Fires would go out, but the gasses would retain such heat that they would reignite as soon as oxygen returned.
Gas fires, however, would stay out, which means every gas-fired furnace and flare would simultaneously snuff out.
In terms of breathing, we probably wouldn't notice. Our bodies can't detect oxygen deficiency, we can only detect high carbon dioxide levels. As long as CO2 isn't building up in our blood, we don't feel like we're suffocating.
Actually, it would be disappointingly undramatic. The phenomenon of a hundred million cars stalling at once would get some attention, and people monitoring air pressure would notice the blip there, but we'd never figure out the cause and eventually just shrug it off.
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Khannan Sundar, BSC Engineering majoring in Computer Science at BITS Pilani, India
Many of the answers to this question will focus on what would happen during those five seconds, but after that, there would be a huge explosion due to the oxidation of all the oceans that have been converted to hydrogen (and any exothermic oxidation). So, the Earth would suddenly become extremely cold for five seconds followed by a Big Bang.