posted by Jasmine .
How do I identify the reducing agent in a reaction of magnesium with oxygen?
Its the one that loses electrons; i.e., the one that is oxidized. To help out a little I can tell you that O2 goes from zero on the left to -2 on the right for each atom. Mg goes from zero on the left to +2 on the right for each atom.
2Mg + O2 ==> 2MgO