Given the fact that the human body is mainly water, and that there are 2 molecules of Hydrogen to 1 of Oxygen in water, how come Oxygen is the most common element in the human body?

Four elements oxygen, carbon, hydrogen and nitrogen comprise 97% of the human body. Oxygen holds first place, making up 65% to 67% of the human body by weight, Because human bodies are mostly water - around 50% to 60% - so most of the oxygen is the "O" in H2O. The weight of a water molecule is 18 atomic mass units with 16 being oxygen and 2 amu being the weight of the 2 hydrogen molecules. Therefore 89% of a water molecule is oxygen.

https://www.livescience.com/common-elements-in-human-body

Oxygen is the most common element in the human body because atomic mass matters more than atomic count when calculating body weight.While it is true that water contains two hydrogen atoms for every one oxygen atom, an oxygen atom is vastly heavier than a hydrogen atom.