Oxygen in humans is used for energy to create nutrients. Oxygen is needed for cellular respiration. Cellular respiration makes ATP. ATP is the main source of energy for cells. It also releases waste products. The mitochondria is the final step in the process and finalizes ATP to move onto glycolysis.
The same process happens in a snake. They have lungs which helps with the process and starts it. The oxygen though in the snake case, is that it helps with giving its muscle energy. Snakes uses its muscle in its mid section to digest food.