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Electrolytic Cells and “Over potential”?

My professor wrote: Eapplied – Ecell= over voltage…

then, he said in an aqeuous NaCl cell, E ovr (H2O/O2) > E ovr (Cl-/Cl2) and therefore Cl-/Cl2 occurs.

What does this mean?

I am very confused!
Also, When choosing, is it best to pick the half reaction with the higher overpotential?
umm, thanks but so not the answer I am looking for….