Firstly, oxygen is not an EDTO critical system. Obstacle clearance driftdown, escape routes, and oxygen endurance is a separate issue.
Secondly, the EDTO area is defined by the
most limiting of the two cases, not the least limiting. For twin-engined aircraft, the most limiting area is usually the single-engined second case. For quad-engined aircraft, the most limiting area is usually the fire suppression first case. A 777 will not be 4+ hours from an available adequate airport, but a 747 might be.
Greybeard, the CP is a preflight calculation based on - largely arbitrary - data and regulations. The PNR is the inflight case based on what you calculate on the spot factoring in all your questions and decisions (what is actually happening).
Happy New Year!
