Oxygen is needed in a process known as Respiration where it is used to produce energy from glucose. This energy is what the cell uses to continue whatever functions that it carries out which are needed for the body to survive. If the cells don't get oxygen, they can't make energy and if they can't make energy, they die.
Some cells are more important than others and so will need a constant supply of oxygen. The brain for instance requires 20% of the oxygen used in the body. If the body suffers from a lack of oxygen for whatever reason, brain cells begin to die within about 4 minutes.
The brain is extremely important as it directs the body. If the cells there begin to die, the body deteriorates and the body stops engaging in certain processes due to a lack of direction from the brain. This coupled with the other cells dying around the body will lead to even more deterioration till the body dies.
