Short answers, pulled from the story.
Carl Wilhelm Scheele produced oxygen by heating mercuric oxide and nitrates in 1771 and 1772, before Joseph Priestley liberated it on the 1st of August 1774. Priority is often given to Priestley because his work was published first, in 1775. Michael Sendivogius had isolated the substance even earlier, before 1604.
Oxygen is a chemical element with the symbol O and atomic number 8, a member of the chalcogen group in the periodic table. It is a highly reactive nonmetal and a potent oxidizing agent that forms oxides with most elements. At standard temperature and pressure two atoms bind into dioxygen, a colorless, odorless gas.
Antoine Lavoisier renamed vital air to oxygene in 1777, from the Greek roots oxys meaning sharp or acid and -genes meaning producer. He mistakenly believed oxygen was a constituent of all acids. Humphry Davy showed in 1812 that hydrogen chloride is an acid without oxygen, but the name had already become established.
Oxygen began building up in the atmosphere about 2.45 billion years ago during the Great Oxygenation Event, roughly a billion years after the first anaerobic organisms appeared. A second event around 500 million years ago, the Neoproterozoic Oxygenation Event, raised levels to near or above those of today.
Every year about one hundred million tonnes of oxygen are extracted from air for industrial uses. The most common method is fractional distillation of liquefied air. Another primary method is pressure swing adsorption, passing dry air through zeolite molecular sieves to deliver a gas stream that is 90% to 93% oxygen.
Smelting iron ore into steel consumes 55% of commercially produced oxygen, while about 25% is used by the chemical industry to make products like ethylene oxide and ethylene glycol. The remaining 20% goes to medical applications, metal cutting and welding, rocket fuel, and water treatment. Oxygen therapy treats emphysema, pneumonia, and some heart disorders.