Oxygen makes up 65 percent of the mass of the human body because at least 60 percent of a normal human's mass is water. Water is made of two hydrogen atoms and one oxygen atom, and the oxygen atom weighs eight times that of both hydrogen atoms combined.
Most of the oxygen at any given time in the human body is bound to water, and water comprises anywhere from 60 percent to 90 percent of individual cells. Just six elements make 99 percent of a human body's weight. After oxygen, hydrogen makes up 10 percent of the human body by weight and carbon comprises 18 percent, nitrogen 3 percent, calcium 1.5 percent and phosphorus 1 percent. The rest of the body's weight is comprised of trace minerals. Carbon is responsible for the organic chemistry in the human body. Nitrogen is part of DNA, and calcium is necessary for bone growth.
Humans breathe in about 19 cubic feet of oxygen per day, which weighs about 1.43 ounces per cubic foot. On average, a human breathes less than 2 pounds of oxygen per day, so the amount of gaseous oxygen in the body at any given time is negligible compared to a 150-pound person who is comprised of nearly 98 pounds of oxygen.