People need Oxygen to breath. Some of that O2 is actually burned with food to produce energy to keep us running.
Food, mostly Carbon and Hydrogen, burn with the O2 to form water (H2O) and Carbon Dioxide (CO2).
a person metabolizes about 0.84 kg per day (0.5-1.3) (1.1 to 2.9 lbs/day) but most of the O2 in the air is breathed right back out.
At sea level the O2 concentrations have an effect on the human body. From NIOSH  we have the following table:.
21% (normal oxygen content in air) = No effect
19.5% (minimum oxygen level for safe entry) = still no effect
16% Impaired judgment and breathing
14% Faulty judgment, rapid fatigue
6% Difficult breathing, death in minutes
These are of course at sea level pressures. With a given pressure of 15 psi, and a concentration of 20%, we get a partial pressure requirement of about 3 psi O2. A bare minimum based on the 16%) is 2.35 Psi PPO2. This works in reverse for lower pressures.
A 5psi cabin should have about 60% O2 for a 3PSI Partial Pressure O2.
Obviously this isn't exact. Denver Colorado is about 12psi (2.4 PPO2) and local concentrations in rooms and such are lower with little effects on acclimated people. (Realize that altitude sickness if pretty common here)
How can the Oxygen be supplied ?
A couple of ways :
Pressurized tank : kinda heavy, but simple and works.
Cryogenic Liquid : much more dense 1:800) so the tank is lighter, It does need to be warmed for use and doesn't store all that well (boiloff)
Solid Chemical : there are 'Candles' that burn giving off O2. can be a fire hazard (like on the Mir space station)
Liquid Chemicals : N2O, and other chemicals can give off O2 when decomposing. can be a fluid handling system instead of a solid.
Endothermic chemicals : Water is a relatively good storage medium for oxygen, provided you have enough electrical power to crack it with hydrolysis. A simple Pop can is a remarkably efficient storage container and water is 16/18th O2.
In reality, a hybrid system would probably be used. A temporary surge tank is a pretty good idea.