When you look up reducing agent, every definition you find uses the word 'substance.' For example, LiAlH4 is a powerful reducing agent, however only the H^- gets oxidized as LiAlH4 reduces some other element in a different compound.
For your problem, let us write the equation this way:
MgO +MgO ---> 2Mg + O2
The O is oxidized from -2 to zero and is the specific atom that is the reducing agent, since it is the Mg that gets reduced from +2 to zero.
However, should we pick MgO as the reducing agent (following the usual book definition) or should we pick the oxygen by itself?
Based on the unusual nature of the problem, I will recommend you pick the O. However, be prepared for it to be the wrong choice and the correct answer to then be MgO.
Source(s):
ChemTeam
