Oxygen makes up around 21 percent of Earth's atmosphere, and, out of the gases found on the planet, humans use it the most, if only because it is necessary for most mammalian life. That said, it finds use in numerous other human endeavors: medicine, construction, transportation and even recreation, to name a few. Swedish chemist Carl Wilhelm discovered the tasteless, odorless gas in 1772 after he experimented, heating different oxygen-containing compounds. While oxygen makes up a good portion of Earth's atmosphere, Earth's crust contains large amounts of it in solid form as various oxides, and the world's oceans contain an abundance of it as H2O, also known as dihydrogen monoxide or water. Oxygen makes up 46 percent of Earth's crust and stands as the most plentiful element in the structure, and it makes up 89 percent of the world's seawater.
TL;DR (Too Long; Didn't Read)
While humans breathe oxygen, the element finds use in many other human endeavors: rocket fuel, fighting cancer, cleaning waste water, recreation, ecological studies and working with steel, to name a few.
1) Humans Breathe Oxygen
Earths mammals, along with many other species, require oxygen to survive. They breathe the gas into their lungs, where the blood absorbs it and sends it to the other cells in the body where it is used for cellular respiration. This is the process by which oxygen and glucose create chemical energy, water and carbon dioxide, which the species then exhales back into the atmosphere. Because oxygen is essential for life, humans store it for emergency uses and in places where breathable oxygen does not occur naturally. For instance, airplanes keep a supply of it on hand in case the cabin rapidly depressurizes (if there's a hole in the plane), submarines keep stores of it that the crew may breathe and hospitals offer canisters of it to patients with respiratory issues like lung cancer.
2) Oxygen for Transportation
While airplanes need air to fly, and while oxygen makes up around a fifth of Earth's air, similarly dense gases could, in theory, provide the pressure necessary for flight. That said, other machines make use of oxygen in some form or another to move. Submarines make use of hydrogen peroxide, H2O2 when starting their engines; prior to this, subs needed to frequently come to the surface to access atmospheric oxygen. Similarly, some submarine torpedos also discharge from these machines using oxygen. Rockets also use liquid oxygen as an oxidizer, rather than as a fuel itself. In short, oxygen works to increase the rate of combustion for rocket fuel. Engines, like the ones in cars, also use oxygen: The element is necessary for combustion.
3) Medical Uses of Oxygen
Many hospitals also keep oxygen in a variety of ways to treat a variety of ailments. Pediatric incubators help provide a safe environment for newborns and infants to develop when they are born prematurely, without the ability to regulate their heat or when they have wounds, to name a few. Doctors regulate the amount of oxygen in these areas as excessive amounts of it can damage newborns, though it is still a necessary element in these scenarios. Oxygen also finds use in other medical areas: doctors enrich gaseous anaesthetics with it to ensure a patient survives, for instance.
4) Oxygen Helps Clean Waste Water
In many cases, water that comes to treatment plants has its oxygen levels depleted. This is something of an issue, considering many of the bacteria and other micro-organisms these plants use to break down harmful compounds in the water rely on oxygen to thrive and outproduce competing, potentially dangerous other organisms in the water. Historically, workers at these plants have used atmospheric oxygen to help these benevolent bacteria, but recently they have started using pure oxygen gas, pumping it into the dirty water and enabling them to use smaller containers than when using the atmosphere's oxygen content to treat the same amount of waste water.
5) Oxygen as Recreation
In the mid to late 2000s, the world saw a rise in the number of oxygen bars. These establishments provide customers with pure oxygen (occasionally "flavored" to some extent), which, these organizations claim, boosts the mood and reduces the stress of a user. Some claim it provides a "high." That said, some government health organizations around the world decry this activity as potentially unsafe, given that the oxygen is not provided in a medical setting or by a medical professional.
6) Oxygen Can Give Scientists a Glimpse Into the Past
Most oxygen has eight neutrons, giving it an atomic weight of 16, but, a far more rare form of oxygen has an additional two neutrons. Called oxygen-18, this heavier set of molecules appears roughly one per every 500 "normal" molecules of oxygen. Scientists can compare the oxygen found in sea water to the oxygen found in glaciers, which formed many eras eons ago. Glacier ice generally has fewer oxygen-18 molecules than seawater, and some scientists theorize they can glean information about previous atmospheric temperatures using the prevalence of oxygen-18 in glaciers: the more heavy oxygen, the cooler the climate at the time the glacier formed.
7) Oxygen Is Cool
Liquid oxygen finds use as a coolant across a number of human efforts. Some computer users who require high processing powers make use of liquid oxygen to cool their rigs. While liquid oxygen also finds use as an oxidizer in rocket fuel, it, too, acts as a coolant in some rocket system. Commercial oxygen coolants also exist for average consumers. Liquid oxygen makes for a good coolant as it has a relatively low density and does not hold heat terribly well.
8) Oxygen: Under Pressure
Many pneumatic systems also make use of oxygen. As compressed gases, like oxygen, are relatively safe and cheap, they find common use in many pneumatic tools, like drills, riveters and paint sprayers. Functionally, pneumatic tools exploit the properties of compressed air, such as pressure, to move various parts of themselves and achieve a goal.
9) Oxygen as an Ecological Indicator
Scientists can study the oxygen content of a lake as a means of determining how healthy it is, or is not. In some cases, such as algal blooms where fertilizer run-off reaches a body of water, the amount of oxygen in said body can be too high, and it ends up harming the ecosystem, in some cases killing fish. However, a certain level of it is necessary for life to grow in water, as many species require it for life, and, in turn, other species rely on them.
10) Industrial Uses of Oxygen
The steel industry represents the largest non-natural use of oxygen. During the forging process, workers blow highly pressurized oxygen to increase the volatile nature, and thus removal, of undesirable compounds in steel. It also finds use in welding, where it can increase the temperature of a flame, allowing it to melt and weld substances that have a higher resistance to heat. Similarly, oxygen-rich air finds use in the creation of acetylene and methanol, among others.