At the sharp end we have a bottle that provides for the flight deck. The on/off valve is to the right of the FO seat. Make sure it's on! The pax access this supply, not by drop down masks as in a jet, but by opening each hatch individually. There are only about 13 masks intotal, so the CC dish them out to whoever needs them most (old, ill etc).
Pax only need oxygen supplies above FL140, so a descent from FL245 wont take all that long.