Here is an example of an O2 leak behind the RH side panel inside the cockpit
On 29 July 2011, a Boeing 777-200 being operated by EgyptAir on a scheduled passenger flight to Jeddah was parked on the departure gate whilst awaiting the last few passengers and completion of hold loading when a fire suddenly began at the lower right hand side of the flight deck. Despite prompt attempts to extinguish the fire, it continued to burn out of control causing major structural damage to the front of the aircraft. However, all 317 occupants were safely evacuated using the still-attached air bridge access at doors 1L and 2L. story here:....http://www.skybrary.aero/index.php/B772,_Cairo_Egypt,_2011
The Investigation could not conclusively determine the cause of the fire but suspected that wiring damage attributable to inadequately secured cabling may have provided a source of ignition for an oxygen leak from the crew emergency supply (the plane was on the ground when it all caught fire)