I landed in Salt Lake City with 200 psi of oxygen left in my tank, so I called for oxygen service. I was cleaning out the airplane when they arrived with a cart full of long green oxygen tanks, so I got to see how this works. I showed them where the access was, a little door under the nose, and watched as they hooked up. I hopped up into the cockpit to watch the gauge, to ensure that it was filling properly.
I hadn't ever watched an O2 fill in progress before. I guess in my head I imagined them having a compressor, or one giant extremely high pressure oxygen tank from which they would charge my oxygen bottle. Or I never actually thought about it. But this being the real world, they have a collection of ordinary cylinders all at different pressures, and they fill by what they call a cascade. You can't ever get all the oxygen from one bottle to another, only equalize the pressure. So they start with the lowest pressure bottle that is at a higher pressure than my tank, and equalize the pressure between those two. Then they shut off the feed and switch to the next lowest bottle that still is at a higher pressure, and so on. They could just equalize my tank with the highest bottle in their array, but then what would they do with all the half-charged tanks?
Somewhere someone has a big compressor to fill their tanks. In case you're interested, an oxygen fill cost me $45 in Salt Lake City. Quite a bargain for about 25 hours of being able to breathe.