Lancer,
Not all airliners use Oxygen generators for passenger Oxygen. Those that do would have to carry supplemental oxygen for flight at 14000.
Oxygen can be a limiting system on some routes. One operator ensures there is enough passenger O2 prior to departure for 4 hours flight at 14000. In terms of EDTO beyond 180 they assume flight beyond 3 hours 45 will be at 10000ft (4:00 hrs minus 15 mins)