Removal of oxygen can be a very effective method of extinguishing fires where this method is possible. Only a slight decrease of the oxygen concentration in air decreases the fire intensity and below 16 % oxygen in the air there is no risk for a fire. Many alternative gases have been used purpose include carbon dioxide, halon and nitrogen. (halon is now disappearing from use, as it is considered a non-environmentally friendly gas).
The disadvantage for all of these gases has been that human beings could be suffocated, if the gas is injected before all humans have been evacuated. CO2 is heavier than air and is often used in building sand other areas where the gas can be contained and the displaced air can raise above the fire.
Nitrogen is lighter than air and is uses for injection where the fire is at an upper surface and the nitrogen can be contained, as it can in a transformer tank. Some manufactures of transformer fire extinguishing systems, have used nitrogen for injection into the base of oil filled transformers to extinguishing a fire burning from the oil surface. In this application nitrogen will stir and cool the oil in the transformer tank and displace the air above the oil and suppress the fire.
Reference: Guide for Transformer Fire Safety Practices (CIGRE Working Group A2.33)