Is it possible for crew oxygen to exacerbate a flight deck fire?
Wouldn't [a] descent to an altitude where you no longer have to use oxygen might be quite an important priority in the event of a [flight deck] fire?
  


It is an important priority. Below is the first page of the procedure for the Airbus A320:



Going through the procedure, if things got out of hand, there is:


  At ANY TIME of the procedure, if situation becomes UNMANAGEABLE :
  IMMEDIATE LANDING............................CONSIDER


And even opening the cockpit window is an extreme option for smoke that is too much to be ventilated.

A note on the depressurized cabin/cockpit scenario: Since there is depressurization, any scenario where a hose leaks, the supply will quickly dissipate towards the pressurization leak source (and not saturate the cabin/cockpit). (It will also help dissipate any smoke.)

So it's the opposite, one would link pressurization with oxygen saturation.

One of the items on the smoke checklist is turning off the air supply systems (called PACKs) and using the manual system to deliberately depressurize the plane:


  PACK 1+2............................................................OFF
  MODE SEL..........................................................MAN
  MAN V/S CTL.............................................. FULL UP


"MAN V/S CTL" means "Manual Vertical Speed Control", basically how the cabin altitude (a pressurization term) is set. Full up means max positive rate, which will equalize the pressure with the outside – dumping pressurized air so to speak.