Air on Earth has a fairly constant composition of 20% oxygen (O₂ gas) and 80% nitrogen (N₂ gas) all over the world and at up to several miles above sea level. This means that for every 100 molecules in the air 20 would be oxygen and 80 would be nitrogen. What does change is the density of the air as you travel up from sea level, the higher you go the fewer air molecules, and so fewer oxygen molecules. This is because as you go higher there is less air above you to push down on you and squeeze the air molecules together around you.

If you are doing any special calculations for a project and you have found a number in a book that tells you a fact about oxygen at sea level you can use that same number for 20 feet above sea level. Any difference will be very tiny.

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.