Mass is amount of matter.
Atoms are little particles of matter. Every atom has its own characteristic mass.
You can measure the atomic mass in any unit of your choice: be it gram, pound or kernels of olives.
For our purpose we select a system where hydrogen atom is assumed to have one (1) unit of mass.

Another fact to note is that all fossil fuel, be it petroleum, coal or natural gas, is hydrocarbons, and that in the burning of hydrocarbons we tie free oxygen to the products of the combustion.

In the above example, combustion of one molecule of propylbenzene (120 atomic mass units) ties 24 atoms of oxygen (having 16 atomic mass units each).

Thus a total of 24 x 16 = 384 atomic mass units of oxygen get tied up in the CO2 and H2O generated by the combustion.

We get that for every gallon of gasoline (6.7 pounds) combusted there’s 3.2 x 6.7 = 21.4 pounds of free oxygen eliminated from the air and engaged in bondage with water and carbon dioxide

Nature can break down the CO2 and water back to their elemental components and release the captured oxygen to the atmosphere. HOWEVER, Nature never equipped itself to break the CO2 and release the oxygen at the rate that we are now tying them up.

One can therefore conclude that before the rest of the world starts mass driving of cars, building power plants and heating homes with gas it would behoove its more advanced societies to strive hard in the quest after non-hydrocarbon-based fuels. 

As the old way is leading into cul de sac, we see global efforts accelerating to solve this fast-becoming acute problem of survival for the inhabitants of Earth.