The oxygen evolution reaction (OER) is a key process in number of electrochemical energy conversion technologies that demands the design of efficient electrocatalysts. In this work, catalytically active gamma-nickel oxyhydroxide (γ-NiOOH) was obtained from a two-dimensional (2D) nickel(II) dithiooxamide Ni(dto) chelate polymer via in situ electrochemical transformation in an acetylene black matrix. The transformation was carried out on the surface of a glassy carbon electrode in 1 M KOH solution, in which the nickel(II) centers of the chelate polymer were transformed into nano-sized γ-NiOOH crystallites dispersed in the carbon matrix. The resultant nanostructured catalyst exhibits OER overpotential of 390 mV (at 12 mA/cm²) and a Tafel slope of 328 mV/dec in 1 M KOH solution. The OER overpotential of the catalyst is lower than that of IrO₂/C electrode, with the same material loading. The oxygen evolution is initiated by adsorption of hydroxide ions at the Ni metal sites of the layered γ-NiOOH, with a subsequent release of oxygen molecules. This study unfolds the potential of in situ electrochemical transformation of 3d transition metal 2D chelate polymers to fabricate active electrocatalysts.