What is the first response in the event of OBOGS failure?

The appropriate response when faced with an OBOGS (On-Board Oxygen Generating System) failure is to PULL THE GREEN RING. This action is critical because pulling the green ring activates the emergency oxygen system, which provides supplemental oxygen to the pilot without delay. This swift response is vital in situations where the OBOGS fails, as it ensures that the pilot maintains adequate oxygen levels necessary for safe operation at altitude.

Maintaining oxygen supply is crucial for preventing hypoxia, which can impair cognitive functions and performance, especially during high-stress scenarios or at higher altitudes. While the other actions might be relevant in a broader context of emergency procedures, they do not address the immediate need for supplemental oxygen as effectively as activating the emergency system.