Question

If an electode reaction has dssolved oxygen as a reactant, is the electrode an anode or a cathode?

Answer 1

The electrode is probably a cathode since most likely there is reduction going on at that electrode.

Elemental oxygen has a 0 oxidation number since `O_2` is a free element. If it is a reactant, there is probably a product containing O. Most compounds containing oxygen have that element in a -2 oxidation state. We can say that oxygen is being reduced, as the element goes from 0 to -2 oxidation state. Reduction always occurs at the cathode in an electrochemical cell.