Citation X (C750)
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe
Flight Crew Landed As Precaution; Flight Crew Diverted; General Maintenance Action
Aircraft
It was the Captain's day to fly. At cruise altitude approximately 90 minutes into the flight; he transferred control of the aircraft to me to momentarily leave the cockpit. As he left the cockpit; I deployed my oxygen mask as a safety precaution. As I put the mask to my face; I noticed the webbing for the head-piece did not inflate; and then that oxygen was not flowing from the mask. I looked at the oxygen gauge and saw that it was at zero. I notified the Captain; who then called the [Chief Pilot] to discuss options. This was a live leg with passengers so we were concerned about recovery flights; not to mention what might happen if we had a depressurization event or smoke and fumes. He placed the walk-around bottle next to his seat; attached his oxygen mask to it; and we began to make plans for the divert into [a nearby alternate].In all candor; we had a departure from the norm with the aircraft but that is what we are trained to manage. I felt we did an excellent job of dividing tasks as we reprogrammed the FMS for the divert; coordinated with ATC; and communicated with [the Chief Pilot] and the passengers. For me; it was a successfully managed event.During the event but before a final divert decision was made; the Captain asked me what I was comfortable with. I told him that though I was confident we could make it to our destination without the need for O2; I was not comfortable at all with that alternative and wanted to divert to the nearest suitable airport. He; [the Chief Pilot]; and I all came to the same conclusion that diverting was the smartest most conservative course of action. Pilot and/or copilot faulty oxygen controllers. I believe we handled the situation as we were trained to do. I also think we did a good job of managing our resources both in the cockpit and with [the Chief Pilot] to decide on the best alternative to flying on to the destination.
CE-750 First Officer reported diverting after noticing his crew oxygen mask was not functioning.