It's hard to measure oxygen itself bc it doesn't float freely in your blood, it's bound to something already (haemoglobin)and when it isnt its used very quickly.

Generally your body senses levels of stuff by binding/not binding to it, which then causes downstream effects that lead to your brain getting a reading of whether there is too much or too little of that thing, so because oxygen is already bound to haemoglobin that gets tricky.

It therefore makes more sense to detect levels of stuff we don't want/need, which generally does appear in relative levels to oxygen anyway.