This process is called oxidation and the substance reacting with the oxygen burns to form an oxide . But it can also produce a slow reaction such as the rusting of iron. Oxygen is taken into the body through the lungs. It is then disolved in the blood and transported to individual cells which derive the energy they need to survive by oxidizing food molecules that are also carried by the blood. But a more reactive and energetic form exists, O3, called ozone.
Ozone is used for purifying water but it is also a major component of smog.
The absorbtion of ultraviolet light by ozone in the upper atmosphere is believed to be responsible for protecting the surface of the earth from that potentially harmful form of light radiation. Exothermic reactions liberate energy. Endothermic reactions absorb energy. Burning of carbon and hydrogen compounds tends to be very exothermic. Both are present in coal and oil which are major energy sources.