Question

why there is more oxygen at the sea level but when we go up the oxygen is less

Answer 1

The % age of oxygen in air remains the same at all altitudes, however due to change in air pressure, the no. of molecules of oxygen in air vary.

At sea level due to high pressure oxygen molecules are closer and hence more in number. At lower pressure molecules are far apart and hence in a unit volume of air less no.of O molecules are present which causes trouble in breathing.