Short answers, pulled from the story.
Oxygen is the third most abundant chemical element in the universe trailing only hydrogen and helium. This element forms approximately half of Earth's crust by weight as various oxides including silicon dioxide iron oxides and silicates.
Joseph Priestley liberated dephlogisticated air on the 1st of August 1774 using sunlight focused on mercuric oxide. Carl Wilhelm Scheele produced oxygen around 1770-1775 but did not publish until years later due to phlogiston theory constraints.
Antoine Lavoisier renamed vital air to oxygen derived from Greek roots acid sharp plus producer begetter because he mistakenly believed all acids contained oxygen. Sir Humphry Davy determined chemists were wrong about this belief in 1812 though the name remained established.
The Great Oxygenation Event began approximately 2.45 billion years ago during the Neoarchean-Paleoproterozoic boundary. Marine photoautotrophs like cyanobacteria provide roughly seventy percent of free oxygen produced on Earth today.
Fractional distillation of liquefied air remains the most common extraction method producing vaporized oxygen leaving liquid nitrogen behind. German engineer Carl von Linde developed commercially viable liquid oxygen production independently in 1895 alongside British engineer William Hampson.