[wkat4242]

Yeah I bet if they could do without the LOX they'd do it too. But for the oxidiser there is no choice but to use a compressed gas, obviously.

[dotancohen]

There are some alternatives to O2, various molecules that contain oxygen which react readily. But those same properties that make them good oxidizers (namely, reactivity) also make them toxic and difficult to handle.

[woooooo]

Liquefied, not compressed. Pressure vessels are heavy, just liquefy it by getting it really cold. It only needs to perform for 10 minutes in booster stages.