The electrocatalytic oxygen evolution reaction (OER) is a critical anode reaction often coupled with electron or photoelectron CO2 reduction and H-2 evolution reactions at the cathode for renewable energy conversion and storage. However, the sluggish OER kinetics and the utilization of precious metal catalysts are key obstacles in the broad deployment of these energy technologies. Herein, inexpensive supported 4 nm Ni-Fe nanoparticles (NiyFe1-yOx/C) featuring amorphous structures have been prepared via a solution-phase nanocapsule method for active and durable OER electrocatalysts in alkaline electrolyte. The Ni-Fe nanopartide catalyst containing 31% Fe (Ni0.69Fe0.31Ox/C) shows the highest activity, exhibiting a 280 mV overpotential at 10 mA cm(-2) (equivalent to 10% efficiency of solar-to-fuel conversion) and a Tafel slope of 30 mV dec(-1) in 1.0 M KOH solution. The achieved OER activity outperforms NiOx/C and commercial Ir/C catalysts and is close to the highest performance of crystalline Ni-Fe thin films reported in the literature. In addition, a Faradaic efficiency of 97% measured on Ni0.69Fe0.31Ox/C suggests that carbon support corrosion and further oxidation of nanoparticle catalysts are negligible during the electrocatalytic OER tests. Ni0.69Fe0.31Ox/C further demonstrates high stability as there is no apparent OER activity loss (based on a chronoamperometry test) or particle aggregation (based on TEM image observation) after a 6 h anodization test. The high efficiency and durability make these supported amorphous Ni-Fe nanoparticles potentially applicable in the (photo)electrochemical cells for water splitting to make H-2 fuel or CO2 reduction to produce usable fuels and chemicals.