@ LEM
You are talking about the "CREW OXYGEN SHUTOFF VALVE" at the P6 Panel.
FCOM says:
"If the
oxygen cylinder is not in the full open position..."
so Terraplaneblues is absolutly right.
@DDG
...Sorry i don`t have diagram`s available on a computer to help explain.
Here they are:
...
@RYR-738-JOCKEY (86 years old???)
"...the green thing" starboard side indicates thought its existence only that the bottle has not discharge the O2 due to overpressure.