Who was the first porson to use oxygen?
How did oxygen become a part of earth's atmosphere and what did this increase in oxygen levels allow?
The majority of oxygen in the atmosphere come from photosynthesis. Aquatic Cyanobacteria (blue green algae) were the first known organisms to use photosynthesis, before there were land plants. The by product of photosynthesis is oxygen. This increase in oxygen eventually allowed organisms to use respiration as a method of energy production. Thus it allowed for land animals.
The simple answer is ... you don't. A majority of scuba dives use compressed air in the cylinders. Therefore you don't need "oxygen" cylinders. HOWEVER, if you are a diver that is diving on Nitrox, in which divers change the amount of oxygen vs nitrogen in their air, then your tank needs to be "oxygen clean" because you are putting oxygen in first, then nitrogen.