The application of the water oxidation reaction to extract electrons from water molecules is important for the future sustainable synthesis of useful chemicals such as hydrogen and organic compounds. Therefore, a cost-effective oxygen evolution reaction (OER) in alkaline, neutral or acidic solution is required, based on the use of catalysts incorporating earth abundant elements. This work demonstrates that β-FeOOH(Cl) nanorod catalysts with high crystallinity and small size (an average diameter of 3 nm and a length of 15 nm) provided the best performance in the OER activity among Fe-based oxide and (oxy)hydroxide catalysts. The pristine β-FeOOH(Cl) nanorods with the high crystallinity are realized by a new process enabling one-pot, fast, and room temperature synthesis, which is the key to forming colloidal suspensions of the highly crystallized pure-phase β-FeOOH(Cl) nanorods. The versatility of this process also enabled doping of a wide variety of transition metals. Doping of Ni²⁺ (at 1.2 at%) improved the OER activity and shifted the threshold potential in the negative direction by 100 mV in an alkaline electrolyte, which was comparable to that of conventional IrO_x colloidal nanoparticles.