Replacing critical raw materials employed in water electrolysis applications as electrocatalysts with earth-abundant materials is paramount for future upscaling to industrial dimensions. In that regard, Ni and Ni-based multimetallic hydroxides, above all NiFe hydroxides, have shown promising performance toward the oxygen evolution reaction (OER) in alkaline conditions. However, it has been shown that the extraordinary performance of these materials is largely due to Fe impurities found in commercial KOH from which electrolyte solutions are prepared. The mechanism of action of these impurities is still not fully understood and, therefore, at the heart of ongoing discussions. In this study, we investigate the OER activity of different nanostructured (Ni_1-xFe_x)OOH samples and find their activities to be influenced differently by the presence of Fe impurities in the electrolyte. From the gathered data, we conclude that the presence of Fe impurities impacts the structure sensitivity of the OER. In purified electrolyte solutions, the OER appears to be a structure-sensitive reaction, while in the presence of Fe impurities, the interaction of the catalyst with these impurities and thus the catalyst surface area becomes paramount.