Oxygen is the most abundant element in the Earth's crust, about 46 per cent by mass, but nearly all of it is locked into silicate rock, where it is doing nothing. Free oxygen in the atmosphere is a different matter, and it is an anomaly.
Oxygen is reactive. Left alone it combines with iron, with carbon, with sulfur, with almost anything, and within a geologically short time it would be gone. That the air is a fifth oxygen means something is producing it continuously. That something is photosynthesis.
For roughly the first half of Earth's history there was effectively none. Cyanobacteria began splitting water for its hydrogen and discarding the oxygen, and from about 2.4 billion years ago the free gas began to accumulate, in what is called the Great Oxidation Event. Banded iron formations, the layered red rock that supplies most of the world's iron ore, are the record of dissolved iron in the ancient oceans being precipitated out by that new oxygen. For much of the life then existing, the change was a catastrophe.
Aerobic respiration, which extracts far more energy from food than any anaerobic route, became possible only afterwards, and every complex organism depends on it. Ozone, a three-atom form of oxygen concentrated in the stratosphere, absorbs the ultraviolet that would otherwise make the land surface uninhabitable, and it too is a product of the free oxygen beneath it.
Liquid oxygen is pale blue and magnetic, clinging to the poles of a magnet, which is a directly visible consequence of the two unpaired electrons in the ordinary oxygen molecule. Medical use of concentrated oxygen dates from the late nineteenth century; industrial use, chiefly in steelmaking, consumes far more of it.