The story of Factorio tells, that the character has to prepare an alien planet for the colonists. But how many planets are earth-like and at the same time have a human-friendly atmosphere? So this planet probably has a poisonous atmosphere and we need oxygen to survive.

How to use the content:

At the start of the game you get 50 big oxygen bottles (50 should be enough). Each big bottle can hold 100 oxygen, so you have to research the production of further oxygen bottles and oxygen quickly.
Your oxygen amount is taken from oxygen bottles in your inventory. If you have no oxygen bottles, you get damage.
The more polluted your position is, the more oxygen you will need!

Gas masks:
As soon as you can craft a modular armor, it's recommended to place a gas mask in your armor. The gas mask will reduce your oxygen consumption.

Oxygen dispensers:
In later game you can build an oxygen dispenser, that will give you oxygen instead of consume the bottles if you keep yourself near the dispenser (how many depends on the distance.
If it has enough oxygen in it's fluidbox, the dispenser spawns an oxygen cloud and players can breathe it's oxygen and refill their empty bottles. Aliens and enemy spawners in near of the dispenser get damage.