It is well known that the “iron” impurity will influence the oxygen evolution reaction (OER) in an alkaline electrolyte, especially for the Ni-based electrocatalyst. Many research studies have investigated the function of Fe in the OER active phase, such as M(OH)2/MOOH (M = Ni and/or Fe), while, surprisingly, very few studies have examined the function of Fe in the “precatalyst” system. Accordingly, in this work, the Ni3–x Fe x P (x = 0, 0.5, 1) series as an Ni-based precatalyst was employed to inspect the function of internal and external Fe in the Ni-based precatalyst system. It was realized that the sample with internal Fe (i.e., Ni2.5Fe0.5P and Ni2FeP) exhibits efficient OER activity compared to that of the Fe-free one (i.e., Ni3P) owing to the large amount of active M(OH)2/MOOH formed on the surface. This indicates that the internal Fe in the present system may have the ability to facilitate the phase transformation; it was later rationalized from electronic structural calculations that the d band center of the internal Fe (middle transition metal) and Ni (late transition metal) holds the key for this observation. Adding excessive ferrous chloride tetrahydrate (FeCl2·4H2O) as the external Fe in the electrolyte will greatly improve the OER performances for Ni3P; nevertheless, that the OER activity of Ni2FeP is still much superior than that of Ni3P corroborates the fact that the Fe impurity is not the only reason for the elevated OER activity of Ni2FeP and that internal Fe is also critical to the phase transformation as well as OER performance.