[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.

[noiseman]

Just a small correction: I was referring to two-place sailplanes when I mentioned the weight. Single-seaters are closer to 600 pounds, and even they have liquid oxygen tanks. There is no difference in this case between a fighter and a sailplane either, because the human still breathes the same.