In battery, overpotential is the potential difference (or voltage measure) between a theoretical or thermodynamically determined voltage and the actual voltage under operating conditions. ... In a galvanic cell overpotential means that less energy is recovered than thermodynamically determined.

How do you determine overpotential?

overpotential = (Experimental - thermodynamic)potential. Say, OER, the thermodynamic potential for OER is 1.23 V (vs SHE) and your obtained OER potential (onset) is 1.63 V, then the overpotential of OER for your material will be (1.63-1.23)V = 0.4 V.

What do you mean by the overpotential of oxygen?

The overpotential is the difference between the theoretical cell voltage and the actual voltage that is necessary to cause electrolysis. It turns out that the overpotential for oxygen is rather high and effectively makes the reduction potential more positive.