Scrubba, for what purpose? There are minimum O2 quantities required for dispatch depending on crew/pax numbers, but only to facilitate descent to 10000'. Oxygen limited routes require more O2 simply because it takes longer to get to 10000' due to terrain.
Sure surplus oxygen may allow a different (to the flight-planned) driftdown/diversion strategy to conserve fuel, but this is an inflight consideration. O2 quantity isn't varied preflight to allow a reduction in fuel burn when CPD limited - more fuel is carried!