Oxygen is the most abundant element in the human body by mass. It makes up roughly 65% of an average human body, largely because the body contains substantial amounts of water and oxygen-rich molecules.
Water alone accounts for much of this result: its formula is H₂O, and oxygen contributes most of water’s mass. Oxygen is also present in carbohydrates, fats, proteins, nucleic acids, and mineral compounds such as calcium phosphate in bones and teeth. It is therefore abundant in both soft tissue and skeletal material.
This answer changes if the question uses a different basis. By number of atoms, hydrogen is generally the most abundant element in the body because water and organic molecules contain so many hydrogen atoms. By mass, however, oxygen ranks first, followed by carbon and then hydrogen in commonly used composition estimates.
Together, oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus account for nearly 99% of human body mass. Nitrogen is essential to proteins and DNA, while carbon forms the structural framework of organic compounds, but neither exceeds oxygen’s mass contribution.