10 meters, not 10 feet.
By the way, guys, I appear to have misread the cite I gave on the mass of atmospheric oxygen. Here’s a more readable one. The mass of atmospheric oxygen is actually 1.2 x 10^15 tonnes, so there is only a thousand times more in the water.
That no one called me on this quite neatly illustrates how bad human intuition is at estimating these sorts of things, I think. You can be off by a factor of a million without anyone noticing. 
Yeah, who cares about 5 or 6 orders of magnitude, anyway?
Actually most of the oxygen in the atmosphere came from carbon dioxide. Although some water was split into hydrogen and oxygen, the hydrogen mostly recombined with atmospheric oxygen before it could reach the top of the atmosphere and escape.
Carbon, in the form of organic compounds, is more persistent, and is eventually incorporated into rocks in the form of kerogen. Photosynthesis has produced more than twenty atmosphere’s worth of oxygen over the lifetime of the Earth, and the corresponding amount of carbon is buried in the crust- and so is most of the oxygen, in the form of oxidised minerals.
Volcanoes constantly emit carbon dioxide, and this is split into carbon and oxygen by photosynthesis. These two elements are removed by the processes of kerogen formation and oxidation of the crust.
If the rate of oxygen removal is low compared to the rate of oxygen production, the atmospheric oxygen level will increase - but this will only go so far, since oxygen is a very active element, and at high levels the rate of oxidation increases considerably - sometimes, I would imagine, in the form of wildfire.
What about all the water vapor, and clouds, in the atmosphere?
I read somewhere that, by volume, oxygen dominates the Earth’s crust (and mantle too, I guess), even more than it does in terms of mass or numbers of atoms. Not only is it the most common element in the crust, but, because, in the compounds it forms, its atoms are electron acceptors, such that the oxygen atoms retain and complete their outer electron shell. Thus the volume of an oxygen atom (including its electron shells) in a crustal compound is considerably greater than the volumes of the atoms of most of the other crustal elements, most of which act as electron donors and so lose their outer electron shell. Walk upon the surface of the Earth and, to a large extent, you are walking on (somewhat impure) oxygen.
According to Wikipedia:
[
](Water vapor - Wikipedia)
At a few percent at most, there’s a lot less oxygen in atmospheric water vapor than there is in gaseous oxygen, so at the level of precision we’re using, it’s not going to affect the thousand to one ratio.
Maybe but this wikipedia reference (^ Morgan, J. W.; Anders, E. (1980). “Chemical composition of Earth, Venus, and Mercury”. Proceedings of the National Academy of Sciences 77 (12): 6973–6977. Bibcode:1980PNAS…77.6973M. doi:10.1073/pnas.77.12.6973. PMC 350422. PMID 16592930.) says the