In this work we report a synthetic pathway to two-dimensional nanostructures of high oxidation state lamellar cobalt oxides with thicknesses of only few atom layers, through the combined use of precipitation in basic water at room temperature and gentle solid state topotactic transformation at 120 °C. The 2D nanomaterials are characterized by X-ray diffraction, nitrogen porosimetry, scanning electron microscopy, transmission electron microscopy and especially scanning transmission electron microscopy coupled to energy dispersive X-ray analysis and electron energy loss spectroscopy to assess the composition of the nanosheets and oxidation state of the transition metal species. We show that the nanosheets preserve high oxidation states Co3+ and Co4+ of high interest for electrocatalysis of the Oxygen Evolution Reaction (OER). By combining high Co oxidation state, surface-to-volume ratio and optimized nickel substitution, the 2D nanomaterials produced in a simple way exhibit high OER electrocatalytic activity and stability in alkaline aqueous electrolyte comparable to standard materials obtained in harsh thermal conditions.