What would happen if oxygen were to disappear for 5 seconds?

That is if either oxygen (O) atoms or only the gas, O2 were to go away, depending on your interpretation.

• Everyone at the beach would get sunburns. Ozone is molecular oxygen, and blocks the majority of UV light. Without  it  we are toast.
• The day-time sky would get darker. With fewer particles in the atmosphere to scatter blue light, the sky would get a bit less blue and a bit more  black.
• 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 (Cold welding).
• Everyone's inner ear would explode. As mentioned, we would lose about 21% 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.
• Every living cell would explode in a haze of hydrogen gas. Water is one third oxygen; without it the hydrogen turns into gaseous state and expands in volume.
• The oceans would evaporate and bleed into space. As oxygen disappears from the oceans' water, the hydrogen component becomes an unbound free gas. Hydrogen gas, being the lightest, will rise to the upper troposphere and slowly bleed into space through Atmospheric escape.
• Everything above ground would immediately go into free fall. As oxygen makes up about ~45% of the Earth's crust and mantle, there is suddenly a lot less "stuff" beneath your feet to hold everything up.

        To sum, it wouldn't be pretty.