  Polar perovskite oxide surfaces are subject to structural reconstruction as a
possible stabilisation mechanisms, which changes the surface structure and
hence the surface chemistry. To investigate this effect, we study the oxygen
evolution reaction (OER) on the reconstructed (001) surface of NaTaO$_3$, by
means of density functional theory (DFT) calculations and compare it with the
non-polar (113) surface of the same material. For the clean surface the
reconstruction has a beneficial effect on the catalytic activity, lowering the
minimal overpotential from 0.88 V to 0.70 V while also changing the most active
reaction site from Na to Ta. Under photocatalytic conditions, the Ta sites are
covered by oxygen adsorbates, rendering a lattice oxygen site on the NaO
terrace the most active with a very low overpotential of 0.32 V. An alternative
surface reconstruction stable in contact with water leads to an oxygen coupling
mechanism with an overpotential of 0.52 V. Our results show that terraced
surface reconstructions enable novel reaction pathways with low overpotentials
that do not exist on other non-polar NaTaO$_3$ surfaces nor on non-polar
surfaces of chemically similar materials such as SrTaO$_2$N.
