Not really much to do with ‘oxygen’ more with density I think you’ll find.
The pitot system for Indicated Airspeed uses the velocity of air ‘particles’ entering the tube to give the ‘pressure’ related to the aircrafts speed. The ‘static’ system is used to compensate for the aircraft altitude. Temperature is corrected using a barometric spring.
What is tricky to compensate for is the ‘density’ of the air. As you climb at a fixed groundspeed the airspeed will seem to reduce as the density of the air reduces thus reducing the ‘pressure’ on the pitot system. A very simplified over view but it gets the point across.
Hence when at 40,000’ plugging along at Mach 0.84 and a ground speed of 550kts your ‘indicated’ airspeed will be in the region of 240-250kts 'ish due to the lack of air density. The system is limiting your indicated airspeed to the max allowable airspeed at the altitude you are at as you will be approaching supersonic flight!
Transition to mach numbers at about 20,000’ and fly a constant Mach 0.84 and your overspeed warnings will disappear as the Mach is a speed relative to the local speed of sound at the altitude you are at.