<p>Thin nanosheets of metal oxyhydroxides (MOOHs) are promising for a range of applications, including electronics, optics, electrochemistry and catalysis. However, their synthesis remains challenging. Here we address this by introducing a topochemical oxidizing approach that enables the production of stable aqueous colloids of four different MOOH nanosheets. Chemically implanted active oxygen species are shown to facilitate the exfoliation of bulk MOOHs with an efficiency strongly dependent on their abundance. Spectroscopic analysis combined with theoretical calculations reveals that alkali cations stabilize negatively charged active oxygen species through electrostatic interactions, regulating the repulsive forces between the layers, and thereby facilitating effective exfoliation. As-produced Cs⁺-CoFeOOH nanosheets have high electrocatalytic performances for the oxygen evolution reaction, achieving 1 A cm⁻² at a cell voltage of 1.62 V in an anion-exchange membrane water electrolyser. This work presents an alternative strategy for exfoliating MOOHs and may open new avenues for fabricating two-dimensional materials from delamination-resistant layered compounds.</p><p></p>