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