The pilots aren't controlling the amount of O2 in the cabin by adding or subtracting supplemental O2 to the incoming air. It's a different mechanism.

Think of a balloon and blow it up with regular air. If the balloon is the same size as your lungs, and you take a breath out of it, you will capture all of the air and therefore all of the O2 in the balloon. Normal breathing.

Now if you take that balloon to altitude, it get's bigger because of the decrease in air pressure outside BUT, there is no more, or less, O2 in there. It's the same exact air. You haven't pumped any O2 in there or taken any out, the O2 molecules are just further apart. So when you take a breath out of the same balloon that has now doubled in size, since your lungs hold the same amount of air, you can only inhale half of the air in the balloon. Now you are breathing the same air as at sea level but only getting half the O2. It now takes two breaths to get the same amount of O2 as at sea level. When there is a decompression, the O2 molecules are so far apart that you can't breath enough air to harvest that loosely packed O2 out of it.

So, in the sense that pilots can control the density of the air in the cabin (pressurization), they can control how much O2 is available to breath by pumping more molecules from outside into the small space of the AC but they aren't adding O2 from a tank.

This is oversimplified and I hope I haven't just made it more confusing.