Why doesn't he immediately inject heroin, cocaine or ecstasy, why waste time on some oxygen? (After reading this question, I really wanted to joke, although I perfectly understand the importance of the question from a scientific point of view!)
The fact is that the entire complex of the nervous system: cerebral hemispheres, trunk, cerebellum, spinal cord is a very delicate and complex structure. After death (biological), a very rapid decay of neurons and connections between them occurs. The cells simply die off. Unfortunately, you cannot save them with oxygen. And other substances will not help either - necrosis is incurable. Moreover, body cells also die off, and the brain does not receive information through feedback channels, losing functionality.
Unfortunately, oxygen is not food for the brain, but only an oxidizing agent for its reactions. Most of all he loves glucose, oxidizing which he gets the energy for work. But that's not all - the brain cannot function fully when cut off from other organs and systems. Imagine that you have lost all your senses at the same time - you see nothing, you don't hear, you don't touch.
The heart-lung apparatus has existed for a relatively long time, and allows you to maintain tissues and organs in a living state - but this does not always mean that a person can wake up or "come to life".