(unburned) oil is why we have free oxygen in our atmosphere

Oil (and the other fossil fuels) wouldn't exist forever on the surface because they would "burn" -- either actually burning or slower "burning" -- forming compounds with oxygen. (Rusting iron is burning in that sense... just very slowly.)

An ancient tree plugged along for years grabbing carbon dioxide, pulling off the carbon atom for building material and spitting out the two freed oxygen atoms.

But then the tree dies and it decomposes, essentially burning in slow motion, taking oxygen back out of the air.

Unless... unless it is buried in silt or volcanic ash or is otherwise kept away from free oxygen. And over time some of it ends up under strata of rock under high pressure and unable to "burn" and it becomes oil.

Oil is a good fuel because it will readily combine with oxygen but it never gets the chance until we bring it up to the surface.

In Richard Dawkins "The Ancestor's Tale" he mentions that the cycle of plant and animal life, including eventual decomposition, is pretty much oxygen neutral. (Trees eat up about as much oxygen in death as they produce in life.) If every living thing had "burned" we wouldn't have hardly any free oxygen at all. But buried hydrocarbons took enough readily burnable stuff out of the system to allow a little free oxygen to exist. (And for animals to develop.)

An implication of that is that if we got all the oil (and coal, natural gas, and other naturally sequestered hydrocarbons) and burned it there would be no free oxygen in the air. It would all end up locked up in more stable compounds.

Ironic implication: we can never run out of fossil fuels because we will be dead from lack of oxygen long before getting the last drop. (Fortunately we cannot get at all the hydrocarbons.)

This post isn't a call to any particular policy. I just think it's just sort of interesting that we can breathe only because a lot of dead lifeforms never got the chance to decompose in air.

Thriving plant life fixed a lot of that CO2 into carbon (increasing the oxygen share in atmosphere).

Before plants could decompose they were covered under earth becoming hydrocarbons.

Thus the "low CO2" plant we have today is the result of millions and millions of generations of plants slowly pulling CO2 out of the atmosphere. If the hydrocarbons are never burned there is no way CO2 concentration will return to primordial earth.

However we are burning hydrocarbons and lots of it. Every year we are "undoing" the carbon fixation which could millions of years to accomplish.

Fish busted out onto land only when there was something up here to breathe. (Something as good as oxygen dissolved in water.)

And all sorts of other forms of fixed carbon that don't involve oil.

Lime was used for light (limelight), but not by burning it. (It gives of a lot of light when heated, like a filiment.)

I don't know if limestone burns (marble does, so at some temperature limestone might) but I don't think exposed limestone grabs a lot of oxygen from the air. Most limestone is pretty stable already containing a lot of oxygen. (Calcium Carbonate = CaCO3)

Crude oil contains only a tiny bit of oxygen -- much less as a percentage than in the atmosphere.

Granted, the biggest factor in what I'm describing is probably coal, not oil, but it applies to all the fossil fuels.