And, even with an O2 mask, extended flight at high cabin altitude can cause deadly decompression sickness.
Back with one of the earlier iterations of Connie Kallita's cargo airline a DC-8 captain made a very bad decision to continue climb to FL330 after a failure to pressurize on takeoff.
From the NTSB narrative:
Witnesses reported seeing the airplane takeoff and they thought the left forward overwing exit was not in place as the airplane became airborne.
According to flight crew interviews and written statements, the F/E was unable to maintain cabin pressurization after takeoff. The first officer (F/O) and F/E, reported there was no cockpit discussion about staying at a lower altitude until the cause of the lack of pressurization could be determined. The captain made a decision to continue the climb. The flight crew donned their oxygen masks and the flight continued. In the vicinity of FL290 both the F/O and F/E reported the captain was not responding to radio calls, however, the captain indicated via hand signals that he wanted the flight to continue the climb. The flight continued to FL330. While at FL330, the captain's condition continued to deteriorate and the F/O took command of the airplane, and requested a descent.
Once the airplane was level at 8000 feet, the flight crew was queried by air traffic if they wanted to declare an emergency. The flight crew declined to declare an emergency and requested to continue the flight to Atlanta, its planned destination. The flight crew then requested to divert to Charlotte. After being informed of the location of the Greater Cincinnati Regional Airport, the flight crew elected to divert there for landing and medical assistance for the captain. The flight landed without incident and the captain was removed and taken to a local hospital.
There were some definite CRM issues with this crew:
In a written statement made after the accident, the first officer stated:
...I took command when I felt that it was necessary without creating a feeling of mutiny. Since Captain XXXX is a strong willed person and when he is in command he is not to be questioned, I followed his orders....
In a written statement made after the accident, the flight engineer stated:
...It is my opinion that the only way to get the airplane to a lower altitude would be to override the captain by use of physical force. When the captain requested higher altitudes, the first officer and I strongly protested, both verbally and by hand signals. These protests were repeated at least three times during the climb...All protests were disregarded by the captain....
https://www.fss.aero/accident-report...report_key=975