How do the big planes (e.g. A380 or 737) flying on high altitudes manage to get fresh air for passengers and crew during the flight?

I know the inner space of the plane used for passengers and crew is sealed and the pressure is maintained within some range.

However in standard flight altitude of about 10-14 km there is not enough oxygen to breathe.  I am not sure whether the standard oxygen ratio is still about 21% and you only have less air (lower pressure).

We talked two possible solutions and maybe I am still wrong:

 1. using some pumps and small pressure tanks to suck in air from outside till we get higher pressure and then use some valves to "replace" the breathed up air with the fresh one

 2. use some pressure tanks loaded with oxygen + some cleaning devices to clean the air and add oxygen to keep the same oxygen level - but this would need to get the oxygen before take-off and such tanks would be really dangerous

So how is it in reality?