A unique core–shell nanostructured oxygen evolution reaction (OER) catalyst composed of an electrochemically inactive crystalline iron oxide core and an active amorphous nickel phosphide shell is presented, and this catalyst results in superior OER activity. Even activators enhancing the activity of the OER catalyst by promoting the redox reactions are reported, but here the exclusive position of iron in the nanostructures indeed boosted the efficiency due to ideal placement. Moreover, these nanostructures are also prepared in a sophisticated mechanistic approach in which selectively one metal is phosphidated and the other is not. Interestingly, in the absence of iron, nickel phosphide crystallized in a different shape, but in the presence of iron, this specifically formed amorphous NixP became more efficient for promoting the OER. Details of the formation of this active catalyst are studied; the electrochemical reactions are investigated, and the OER activity is compared with that of different leading metal phosphides.