Oxygen is vitally important to us; not just because we need to breath it but because of how versatile and widely used it is:

• Everyone would get sunburnt as oxygen makes up the ozone and normally helps to block out UV light.
• The sky would get darker during the day if there were fewer oxygen molecules to scatter blue light; the sky would appear less blue and more black.
• All engines that use internal combustion would stop working as oxygen is fundamental for combustion.
• All pieces of untreated metal would suddenly weld to one another, as usually the oxidation layer on the metal prevents this.
• Your inner ear would explode as you would lose 21% of the air pressure in an instant, as if you had been suddenly transported to high altitude.
• Any building made of concrete would crumble as oxygen helps to bind all concrete structures.
• Water is one third oxygen, without it the Hydrogen becomes a free gas and expands, thereby destroying all living cells and evaporating the oceans.
• The earth below us would disappear and we would free fall. The earth’s crust is made up of oxygen, about 45% so without it the majority of the earth’s crust would disappear from beneath our feet.

References: http://www.breathing.com