Nickel–iron oxyhydroxide (Ni–Fe–OOH) catalysts have been widely studied for their high activity for the oxygen evolution reaction (OER). Here we demonstrate improved OER activity through incorporation of a third cation, aluminum. Atomic layer deposition (ALD) was used to deposit thin films of nickel oxide (Ni–O) and nickel–aluminum oxide (Ni–Al–O) to measure activity for the OER. Electrochemical preconditioning of the oxide films led to the formation of the OER-active oxyhydroxide catalysts. For Ni–Al–O films, electrochemical preconditioning resulted in aluminum dissolution until a stable composition at a Ni:Al ratio of 9:1 was reached. Because iron can be incorporated into the films from the solution, compositional effects of iron were studied by controlling the iron impurity level in the electrolyte. Turnover frequencies (TOFs) were determined for each catalyst, and it was found that the highest performing electrocatalysts were the films containing nickel, aluminum, and iron, confirming that aluminum exerts a promotion effect on nickel oxyhydroxide catalysts. Studies showed that unlike the Ni–Fe–OOH films, for which the TOF had very little thickness dependence, the activity of Ni–Al–Fe–OOH catalysts was dependent on thickness. This effect may arise from morphological changes in the catalyst film that modulate the density of the active site with thickness. For the thinnest films, aluminum doping improved the TOF of Ni–Fe–OOH catalysts by over 3-fold.