The O2 crew oxygen is out, a leak of some sort. There is no further problem at this stage ie pressurisation etc is normal. We get a couple of portable O2 bottles on the flight deck that give us O2 in our case for 1:17 -2:35 depending on Hi or Low flow. So we are not out of O2, and at this moment in time we do not actually need it….so why descend to FL250? And is so why 250 and not FL100?
If I descend to 250 I will burn a heap more fuel than at altitude and might now be fuel limited and need a tech stop, whereas if I stay at altitude I can make destination.