The same amount through out the entire bottom 10 km of the atmosphere DUE TO MIXING BY TURBULENCE... the composition of the atmosphere is for all intents and purposes the same everywhere on average.
About 20% of the total atmosphere, essentially the same as at sea level. In order to get to having half the amount of oxygen at sea level, you would need to get to about 20,000 feet.
Oxygen makes up about 20.9% of the volume of air at low elevations. It should not be much different between sea level and 20 feet above sea level, as gasses can move around freely.