Oxygen, a nonmetallic chemical element that is contained in air and water, in the earth's crust, and in all forms of life. At ordinary temperatures, free (uncombined) oxygen is a gas with no odor, taste, or color. All animals and most plants require free oxygen to live—without it, they would suffocate. 
Oxygen is a very reactive element. We breath it and life depends on it. Oxygen makes up part of water, the other part is Hydrogen.