What will happen to the environment and atmosphere. It is the most important gas. Imagine a world without oxygen for 5 seconds. It may sound easy for you since many of us can survive without oxygen for more than 1 minute. But can our nature and neighboring things can exist without oxygen for more than 5 seconds.
To this our answer may be yes but actually no one can survive without oxygen even for more than 1 second. If our world is left without oxygen even for 5 second, all metals articles will infuse together and bind with each other.
Take away the oxygen and our earth would become most dangerous to live in. You may suffer from severe sunburn as without oxygen we will be directly prone to the UV rays of the sun as Ozone layer won't be there since ozone layer is made up of oxygen and absence of oxygen will lead us to directly prone to the harmful effects of the sun's UV rays.
In addition to the sun burn our inner ear will explode. Losing oxygen means losing 21% of the sir pressure as earth contains 21% oxygen all over. And change in the air pressure our ears wouldn't have enough time to adapt to the present situation.
Without oxygen there will be no fire. And all the transport system would collapse as our the vehicles runs on the combustion ignition process. And without oxygen there will be no vehicles as well. As all the vehicles of the world will stop as and where it is. Any non electric mode of transport will fail. Planes will fall from the sky and millions of cars will stop.
The sky will appear dark. As when there is no oxygen there will be less things to bounce in the sky and hence that will make the sky dark as the sky of the space. It is also dark since there are no particles in the sky to bounce.
Concrete structure will collapse as oxygen is the main agent that binds cement with the concrete. And absence of oxygen will lead to all the structures of the world to come down.
And last but not the least the earth crust will crumble as the earth crust is made up of 45% oxygen. The crust will crumble and will continue to crumble until there is nothing left making you as well as other things to crumble and fall.
But luckily this will never going to happen and you can take a deep breadth and relax.