Vorab vielleicht: SERA und Part-OPS haben nichts miteinander zu tun...

Wenn es Dir wirklich nur um die Legalität geht: Es gibt NCO keine harten Limits bezüglich O2, sondern der PIC entscheidet. Bei Ramp Checks brauchst Du also nichts zu befürchten:

NCO.OP.190 Use of supplemental oxygen

(a) The pilot-in-command shall ensure that all flight crew members engaged in performing duties essential to the safe operation of an aircraft in flight use supplemental oxygen continuously whenever he/she determines that at the altitude of the intended flight the lack of oxygen might result in impairment of the faculties of crew members, and shall ensure that supplemental oxygen is available to passengers when lack of oxygen might harmfully affect passengers.