Without food, your body would die in a matter of weeks. Without water, you would day in days. How long do you think you would live without oxygen? As an experiment, see how long you can hold your breath. How quickly does your body let you know it needs more air?
Most lifeforms use oxygen as the main ingredient in many of the chemical reactions needed for life. This is because oxygen is a very efficient element. The availability of oxygen in the Earth's atmosphere is believed by biologists to be a major reason for the advancement of life on Earth. Without it, life would probably still be very small and very slow.
Organisms get oxygen from their environment in a variety of ways. Many land animals breath oxygen directly from the air, while ocean-bearing animals often use the oxygen dissolved in the water to survive. No matter how they get it, oxygen is an important need for almost all lifeforms.
