Question

What happens if the level of oxygen decreases from21% to some smaller amount​

Answer 1

What would happen if there is more oxygen? Animals would become more active and flourish. As a result, plants would be eaten more and eventually disappear. At that point, all animals would suffer and disappear too. The situation is the opposite of Paleozoic times when oxygen were less abundent and carbon dixide were predominent. The whole earth were covered by the greens and animals struggled to survive. Afterwards, when the greens produced enough oxygen, animals began to flourish. So, you see, our earth is a very delicate ecological system, where the balance between oxygen and carbon dioxide plays a critical role.

Answer 2

If you mean it we lose 15% of current concentrations, to %17.85 it will be harder to breath we will have a harder time concentrating and thinking. Many more people would need to be put on Oxygen to survive, many smokers and others with lung and blood issues would likely die before they knew what was wrong. Some of it might depend on what replaces that missing %3.15 if it disappears because of combustion and is replaced by CO2, while not immediately fatal, combined with the lower Oxygen content could be devastating to most animal life. The healthiest would likely be OK, and those living in very green areas, forests, crop fields etc would do better (assuming the sun is out and plants are doing their thing, if it was a large fire that caused it, there might be no sun for a while, so the plants would be sucking up Oxygen too).

if you mean total Oxygen drops to %6 then those without access to Oxygen masks are pretty much goners.