Question

Given what you know about the human body, what do you think accounts for the high percentage of oxygen (65.0%)? given what you know about the human body, what do you think accounts for the high percentage of oxygen (65.0%)?

Answer 1

Knowing the fact that around sixty percent of the human body is water, which consists of two hydrogen atoms and one oxygen atoms, and that other biomolecules like proteins also have oxygen atoms in their structure it's not a surprise that the percentage of oxygen in the human body mass is that high. 
Even though there are two hydrogen atoms and only one oxygen atom in the water molecule, the oxygen atom has 16 times more mass than the hydrogen atom and therefore has a bigger contribution to the overall body mass.