the amount of oxygen we have used is minuscule compared to the amount in the atmosphere.

The mass of the atmosphere is about 5.5 quadrillion tons and 20% of that is oxygen.

You need to remember that it is only if we use it with carbon that is not part of the carbon cycle on earth that it is a net increase. So if we burn a tree for fuel and then a new tree grows up and replaces it the same amount of oxygen is released from it photosynthesis.

So it it primary fossil fuels from the ground that use up oxygen and produce CO2. The amount of CO2 is today 412 PPM an increase by 47% from the preindustrial age. let's say 200PPM.

1 PPM =0.0001% so an increase be 0.02%. Compare that to the atmospheric oxygen level of 20.95%. The result is we have used around 1/1000 of the oxygen in the atmosphere since the start of the industrial revolution