What is the oxygen content of dry air in the atmosphere?

By volume, dry air typically consists of 78.09% nitrogen, 20.95% oxygen, 0.93% argon, and 0.04% carbon dixoide. Usually, air also contains small but variable amounts of water vapor, which takes up on average 1% of volume at sea level.
Except for water vapor, these values stay mostly constant until you reach an altitude of $10$ km.