[joshua c]

dont they have alarms if O2 drops?

[rdcbn1]

It's a cabin pressure issue. At 30,000 ft, if the plane depressurizes the low partial pressure of oxygen gives you about 90 seconds before becoming unconscious.

The pilots have emergency oxygen masks in cockpit that they put on immediately if there is a depression. They then put the plane into a rapid descent to get below 10,000 feet as soon as they can.

[MortMan]

O2? Not that I am aware of. But pressurization drops are alarmed. It just depends on the flight crew getting their masks on in time. If they don’t - with a locked cockpit door - the flight is over when the fuel runs out.