Has the burning of fossil fuel, along with deforestation, urban expansion, and other things that reduce the amount of CO2-eating plants, reduced the amount of atmospheric oxygen yet? I assume it will eventually, or will algae blooms take up the slack? How far are we from a tipping point if we continue to “drill, baby, drill?”
Never. CO2 famously just passed 400 parts per million (ppm) of the atmosphere. Oxygen is 209,460 ppm (~21%) of the atmosphere. Even if we reach something crazy like 1600 ppm of CO2, which is well beyond what anyone’s estimating, that would reduce oxygen all the way down to 208,260 ppm. There just isn’t enough fossil fuel to use up a substantial portion of the oxygen in the atmosphere.
And I suppose we’ve burnt most of it already. That’s a very minor load off my mind, and wrecks any potential to use in in a SF story set on Earth. Thanks!
IIRC we’ve burned about 5% of the world’s estimated fossil fuel resources and about 20% of the amount the we can recover, with energetic gain, using current technology.
So no, not even close to having burned most of it. Not even close to having burned 25% of it.
So there’s still hope! Get my agent on the line!
If I had an agent. 
Even using Blake’s numbers, which he didn’t cite, we can burn at most another 19x what we’ve already burnt. Given that we’ve already gone from 280 ppm to 400 ppm of CO2 (a 120 ppm increase), we could get another 2280 ppm increase or so, which is an equivalent decrease in oxygen. Actually, I think most estimates are that about half of that is going into sinks like the ocean, so let’s estimate double that - 4560 ppm. That’s getting all the way down to 204,900 ppm of oxygen!
What about the water produced by burning fossil fuels? Burning any hydrocarbon produces both CO2 and H20.
We already have a large water sink at equilibrium with the atmosphere. It’s called the ocean. Adding more water won’t make any difference to atmospheric water levels, and the amount in fossil fuels won’t lead to sea level rises.
I think his point is that it would consume oxygen, which is true, and not something I had accounted for. Exactly how much depends on the fuel.