Regarding oxygen leaking: pressurising airlocks requires oxygen, the larger the room the more oxygen "wasted" each de-pressurisation. Try to keep your airlocks as small as possible to alleviate this.
- Just tried testing this with a derelict and it doesn't seem to be a case. If I sealed off its oxygen supply and placed a beacon inside, then the total amount of O2 seemed to stay the same both before and after de-pressurization. Results were the same when I increased the area of the airlock to 5x5.