Question

When molecular oxygen (O2) is converted to water (H2O). Is O2 being oxidized or reduced.

Answer 1

The element oxygen gets hydrogen added to it

Since reduction is defined as the addition of hydrogen or the loss of oxygen (in this case the addition of hydrogen), O2 is being reduced.

Answer 2

I agree but I don't like to use the definition with hydrogen because redox can take plce without hydrogen being involved;. I prefer to use the definition of
Oxidation is the loss of electrons.
Reduction is the gain of electrons.

In O2 ==H2O, O2 has an oxidation state of zero and O in H2O has an oxidation state of 2-. To go from 0 to 2- is the gain of electrons; therefore, that is reduction.