a person requires approximately 1 kg of oxygen a day.

how to calculate this: a person requires roughly 8 MJ of chemical energy a day, which is about 0.5 kg of bread. to oxidize this in a 2:1 ratio of oxygen:carbon (CO2), you need 1 kg of oxygen.

so if you use some kind of face mask to supply you oxygen, you might inhale 200 mbar of pure oxygen. however, the body cannot take that all in all at once, so the air you exhale has like 160 mbar of oxygen still (that the body could not absorb) and 40 mbar of exhaled CO2. at least that's how it works on Earth. (i should link a source here, but i'm too lazy to look one up, i write this down from memory)

so inhalation absorbs about 20% of the available oxygen. so if you require to take in 1 kg of oxygen in a day, that means that 5 kg have to be provided in total. so how big would the flasks have to be?

if a flask is 1 liter in size, it would contain 1 g of oxygen at 1 bar, but if we increase the pressure, it can hold more. obviously, we can't increase the pressure to 5000 bar to make it hold 5 kg of oxygen at once, because the highest pressures that can be safely used in industrial settings are 200 bar, and that's already incredibly high. so if we only put 50 bar of pressure into the flasks, one flask would hold 50 g of oxygen, which is about enough for 15 minutes, before you have to change it again. (we need 5000 g of oxygen supply in a day of 86400 seconds, so roughly 50 g supply oxygen for 864 seconds which is almost 15 minutes).