There is always oxygen in the Earth's atmosphere. However, the concentration of oxygen decreases as you move higher up in the atmosphere. This is because the mass of the air exerts a pressure that compresses the lower layers of the atmosphere, making them denser. The higher up you go, the less dense the air becomes, and the concentration of oxygen decreases.

At sea level, the concentration of oxygen is about 21%. At the tropopause, which is the boundary between the troposphere and the stratosphere, the concentration of oxygen is about 10%. At the mesopause, which is the boundary between the mesosphere and the thermosphere, the concentration of oxygen is about 0.1%.

While humans can breathe the thin air at the tropopause, they would quickly suffocate at the mesopause. This is why astronauts need to wear oxygen masks when they are in space.