A mix of 79 % nitrogen and 21 % oxygen at a pressure of 1 atm, 14.7 psi or 1.01 bar is more fireproof than pure oxygen at a pressure of 0.21 atm, 3.087 psi or 0.212 bar.

The nitrogen content cools the fire. There are poorly flammable substances that would sustain a fire in the pure oxygen at the low pressure but extinguish by itself in air.

A mix of 79 % nitrogen and 21 % oxygen at a pressure of 1 atm, 14.7 psi or 1.01 bar is more fireproof than pure oxygen at a pressure of 0.21 atm, 3.087 psi or 0.212 bar. The partial pressure of oxygen is the same for both gases.

The nitrogen content cools the fire. There are poorly flammable substances that would sustain a fire in the pure oxygen at the low pressure but extinguish by itself in air.