O2 comes from a physiological plant process called photosynthesis. Plants take in sunlight energy, water (H2O) and carbon dioxide (CO2) and produce oxygen and sugar.
O2 has a molecular weight of 32 atomic mass units (amu) and a density of 0.089212 pound per cubic foot, according to Universal Industrial Gases.
At a concentration of 21 percent, oxygen is the second most abundant compound in the earth's atmosphere---behind nitrogen---according to "USA Today." This concentration stays the same throughout the atmosphere, but there is less oxygen by mass as altitude increases.