Density functional theory (DFT) calculations are performed to study the oxygen evolution reaction (OER) on tungsten oxide (WO3) surfaces. The free energies of the proton coupled electron transfer (PCET) steps are calculated and from these the OER overpotential is calculated as the characteristic parameter for the electrochemical activity. The effects of surface orientation, oxygen vacancies, and doping by Chromium (Cr) and Molybdenum (Mo), on the OER activities are analyzed. The difference in OER overpotential for the three surface orientations, (200), (002) and (020), is found to be very small (0.07 V). The presence of oxygen vacancies in the first WO3 sub-layer do not favor OER, while vacancies in the second sub-layer do reduce the OER overpotential. A volcano plot with all overpotentials calculated in this study shows that the OER is favored at the Mo doped (200) surface (reactions happen at the Mo site). This paper demonstrates that DFT calculations of the electrochemical activity are an efficient method to identify active surfaces and can therefore save a lot of experimental effort in designing more efficient photoelectrodes for water splitting.