Crew Oxygen A320
 

If during cruise phase (say at FL350) one notices on the DOOR System Display page that the crew oxygen has suddenly depleted what action is recommended? Any suggestions? Thanks.

Descend... but your airline might not be too happy you doing that.

Start a quick descent to a lower level whilst getting some of the portable O2 bottles up front. Once you have them maybe level off at FL250 and decide on the plan. Think FL250 is the limit for non quick donning masks.

Hi,

You will first get an ECAM advisory condition when the pressure reaches 800 PSI, apply the actions first (check that the mask is correctly stowed basically). Check then the depletion rate and compare it with the FCOM LIM section of Oxygen to find out the minimum value required for your flight and hence how much time you still have to make a decision. In the mean time a very good suggestion as given before to get some portable oxygen in the cockpit. All flight crew member must must use supplemental oxygen if the cabin altitude exceeds 13000 ft, the 25000 ft rule relates to first-aid oxygen for passengers.

Crew Oxygen
 

As far as I know there is no ECAM Advisory for low Oxygen

There is, check QRH - ABN - 80 - ECAM advisory conditions. On the A320 you will get an advisory at less than 600 PSI and as said above should check if the masks are stowed correctly. Depending on leak rate that won't give you all that much time to decide what to do.