The oxygen that is found is the oxygen canisters have been filled into the canisters at extremely high pressures. This is the purest form of oxygen depending on what is needed to be used for, for instance medical reasons. The canister can be half empty and the high pressurised oxygen can have more space in the canister and also no it is never half empty. This is because any gas can fill up the empty space in the canister. A vacuum or complete absence of gas is only achieved artificially, therefore if not oxygen some other gas must take up the empty space.