What is the relationship between altitude and air pressure and amount of oxygen?

The higher you go, the lesser the air pressure becomes. This can be logically understood. Atmospheric air pressure is caused by the weight of the air above it. At greater heights, the amount of air above you is less, so there is less weight pressing on the air you are in. This is the reason why the atmospheric pressure is higher at the sea-levels and lower at mountains and places of high altitudes.