The oxygen tanks are part of the pre-flight checklist. If the plane doesn't have a minimum number of oxygen tanks working (the number varies by type of aircraft), it can't fly. Same for the defibrillator.

Let's keep in mind that the initial report of the allegedly empty oxygen tanks and broken defib (AED) came from the relatives, who probably weren't in the best frame of mind over this. From what I've read about AEDs, they don't work in all situations. It depends on the rhythm of the heart, according to this from one of the doctors on this flight:

Two emergency medical technicians performed cardiopulmonary resuscitation on Desir. Shulkin said he assisted with the use of a defibrillator, but that it wasn't successful because once the pads were applied, the rhythm of Desir's heartbeat "was not consistent with one that would respond to a shock."

Complete article here