Upon OBOGS failure, what is the recommended action regarding the OBOGS supply lever?

When there is an OBOGS (On-Board Oxygen Generating System) failure, the recommended course of action is to set the OBOGS supply lever to the OFF position for both sides. This is crucial because continuing to use OBOGS in a failed state could pose serious risks such as contamination or incorrect oxygen levels, which may lead to hypoxia or other adverse physiological effects.

Setting the lever to OFF helps ensure that the system is not attempting to operate under faulty conditions and prevents the introduction of potentially harmful air into the crew's breathing supply. This action also allows for the crew to switch to an alternate oxygen source if necessary, facilitating safer conditions for the pilot and crew.

While other options like keeping it on, or setting it to maximum or half might seem like ways to maintain oxygen supply, they do not address the essential safety protocol needed during a failure and can lead to further complications. Hence, turning the OBOGS supply lever OFF is the safest and most effective response during an OBOGS failure.