Oxygen is the 8th element in the periodic table according to its atomic number. Oxygen is a non-metal and exists in gaseous state in normal conditions. Oxygen is a source pf combustion; no object can be burned or comusted if there is no oxygen. Oxygen is also necessary for the life process of digestion. Oxygen's atomic mass is 15.99999 u approx.
Oxygen can be created(or rather extracted) in many ways. There is no actual known information about the original source of creation but oxygen can be found in medievally new star system of the universe. Oxygen that is on earth was not really formed from the primordial nebula, from which the solar system was created. Oxygen was not so important for existence of life in earth, it was methane. Oxygen was created by the first living microbes who consumed methane together with H2O and exhaled out the pure oxygen gas. Also there are theories that many other events occured, such as asteroid impacts, that in brought in some more oxygen. This great amount of oxygen release resulted in about 80% earth to be covered in oxygen. But now the levels of oxygen are decreasing. Currently earth has about 20.7% oxygen