Depends on what rules you have to use. Under FAR135 (the ones I'm most familiar with right now), up to 15,000' cabin altitude you have to have enough 02 for 10% of the pax (rounded up to the nearest whole number), + crew for the entire time for flight between A120 & A15 (also crew O2 for any flight >30mins between A100 & A120).
Allowing for all occupants to use O2 during the descent, crew to use 02 for the entire divert time until below A100, and the 10% pax bit, there will be a maximum range you can fly at A150 for the available 02.
02 may or may not be the most limiting range. You also have to consider other possibilities eg 1 inop fuel needed with driftdown to 1 inop altitude, and also the fuel needed at the O2 altitude.
Or how about gear extended range? If you can't get the wheels to retract then you'll be limited to a lower altitude (and speed!), and probably also have a higher fuel burn at that speed compared to cruise.
They're all independent, but one of them will be the most limiting.