I speak here from a position of ignorance. In the aviation video linked below, a pilot assured himself that his oxygen levels were fine with an oximeter and passed out due to CO shortly thereafter. His autopilot kept the plane level and the unpiloted descent to the ground resulted in a survivable skidding stop.
It turns out oximeters can not tell if your hemoglobin is saturated with CO or O2. I could easily have made the same mistake.
https://youtu.be/6ebhig-vr9o?si=tay7sPNzkZK4pP-F