[Islander7]

A decent-sized bottle will supply oxygen for several hours...

A decent sized bottle lasting several hours would weight 300+ pounds and be the size of a large bomb. Not an option.

[noiseman]

That's just not true. I used to fly sailplanes, and the bottles that we had installed would last for several hours, and were connected to pressure breathing regulators that were good to over 40,000' (for high-altitude flights in mountain wave). I don't know what the bottles weighed, but it was nowhere near 300 lbs, as the empty weight of most sailplanes is around 800 to 900 lbs.

I'm also pretty sure that's how most fighters have handled it as well until recent generations. In fact, I just looked it up and the F-18, for example, used liquid oxygen (bottle) until recent variants that upgraded to an oxygen generator.