* CO2: Each CO2 molecule has 2 oxygen atoms.

* H2O: Each H2O molecule has 1 oxygen atom.

Calculations:

* Oxygen in 2CO2: 2 molecules * 2 oxygen atoms/molecule = 4 oxygen atoms

* Oxygen in 3H2O: 3 molecules * 1 oxygen atom/molecule = 3 oxygen atoms

Total: 4 oxygen atoms + 3 oxygen atoms = 7 oxygen atoms