Based on numbers, O2 concentration is probably not a good indicator.

Clean air contains about 20.9% O2 and 0.04% CO2. At 2000 ppm CO2, which according to the author is bad enough to impair judgement, that's 0.2% CO2, it that CO2 is the result of respiration, it means that about 0.2% O2 was consumed, so that's a drop from 20.9% to 20.7%, a very small difference. 20.7% is not low enough to have a significant effect, the CO2 itself is the problem, not the drop in O2. And using O2 concentration as a proxy for CO2 doesn't look reliable to me: the difference is small and other things, like humidity can affect O2 concentration.

As for the sensor, O2 sensor in cars compare the O2 concentration between the outside air and exhaust gases, it needs outside air as a reference, but what you are measuring is the outside air itself, you don't have that reference.

>that's 0.2% CO2, it that CO2 is the result of respiration, it means that about 0.2% O2 was consumed,

I dont know anything about human respiration, but I know a little about chemistry and theres no reason to assume this is true. Basic stoichiometry.

According to a random article on the internet[1], nominal co2 production is 80% of oxygen consumption.

Your point appears broadly correct, just wanted to point out some faulty reasoning that could lead to incorrect results in the future.

[1] https://societymechanicalventilation.org/wp-content/uploads/...

> O2 sensor in cars compare the O2 concentration between the outside air and exhaust gases, it needs outside air as a reference

Source?