He had produced oxygen gas by heating mercuric oxide and various nitrates by about 1772. Scheele called the gas fire air because it was the only known supporter of combustion, and wrote an account of this discovery in a manuscript he titled Treatise on Air and Fire, which he sent to his publisher in 1775.
https://en.wikipedia.org/wiki/Oxygen
For example, at sea level (1 atm or 760 mmHg) and 4°C (39°F), 100% air-saturated water would hold 10.92 mg/L of dissolved oxygen. ³ But if the temperature were raised to room temperature, 21°C (70°F), there would only be 8.68 mg/L DO at 100% air saturation ³.
http://www.fondriest.com/environmental-measurements/parameters/water-quality/dissolved-oxygen/
While water equilibrates toward 100% air saturation, dissolved oxygen levels will also fluctuate with temperature, salinity and pressure changes ³. As such, dissolved oxygen levels can range from less than 1 mg/L to more than 20 mg/L depending on how all of these factors interact.
http://www.fondriest.com/environmental-measurements/parameters/water-quality/dissolved-oxygen/
The mean DO level for adult salmonids is 6.5 mg/L, and the minimum is 4 mg/L ¹². These fish generally attempt to avoid areas where dissolved oxygen is less than 5 mg/L and will begin to die if exposed to DO levels less than 3 mg/L for more than a couple days ¹⁹.
http://www.fondriest.com/environmental-measurements/parameters/water-quality/dissolved-oxygen/
Yet too much carbon is also disastrous to life, so we should be appreciative that the Earth seems to have just about the right amount. Venus has about 100 times as much carbon and suffers from the runaway greenhouse effect that has produced a surface temperature of about 750 K compared to about 300K for the Earth.
http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/atmohab.html
By mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O. 2.
https://en.wikipedia.org/wiki/Oxygen
Animal life as we know it requires an atmosphere with sufficient oxygen. In order for plants to carry out photosynthesis, there must be a sufficient amount of carbon in the atmosphere in the form of carbon dioxide. For sustaining a stable habitable temperature, the mixture of greenhouse gases is critical.
http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/atmohab.html
The amount of dissolved oxygen needed varies from creature to creature. Bottom feeders, crabs, oysters and worms need minimal amounts of oxygen (1-6 mg/L), while shallow water fish need higher levels (4-15 mg/L)⁵. Microbes such as bacteria and fungi also require dissolved oxygen.
http://www.fondriest.com/environmental-measurements/parameters/water-quality/dissolved-oxygen/
At low earth orbit altitudes, sufficient atomic oxygen is present to cause corrosion of spacecraft. Oxygen was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, and Joseph Priestley in Wiltshire, in 1774, but Priestley is often given priority because his work was published first.
https://en.wikipedia.org/wiki/Oxygen
An Oxygen Atmosphere for a Habitable Planet. Earth's oxygen content is 45% and 85% by volume according to Ward & Brownlee. This compares with about 0.1% in the Sun, the so-called cosmic abundance. But having a lot of oxygen on a planet doesn't imply that there is an oxygen atmosphere.
http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/atmohab.html
how much oxygen is needed to support life
Bottom feeders, crabs, oysters and worms need minimal amounts of oxygen that is 1 to 6 milligrams per liter, while shallow water fish need higher levels that is 4 to 15 milligrams per liter rest to 5.