Let me try to add some commen sense to this discussion:
1. The air (oxygen) the passengers breath on all current airliners comes for outside the aircraft. It enters the passenger cabin through the engines as bleed air. From the engines goes through a series of coolers and heaters that provides the passengers with breathing air, heating air, air conditioning and cabin pressurization. Even if all the engines fail, as long as the engines turning, you will still have air to breath and pressurization, even though the heat and air condition systems may be lost. So there is no way for the passengers to run out of oxygen.
2. There is a second source of air (oxygen) which is only used in emergancy, like when there is rapid decompression or a failure in the bleed air system and the cabin pressure can not be controled. There are two types of these systems. The hard plumbed system, were oxygen lines are ran from central tanks to individual masks for the passengers and crew. Or chemical oxygen gererators which are provided for each passenger seat, in the lavs and for the cabin crew.