The lowering of reaction overpotentials is a persistent and universal goal in the development of catalysts for (photo)electrochemistry, which can usually be facilitated by selectively stabilizing one reaction intermediate over another. In this mechanistic study of the oxygen evolution reaction (OER) catalyzed by cobalt-intercalated layered MnO2, we show that confinement effects and local cobalt atomic ordering in the interlayer space can be synergistically used to tune the adsorption energies of O, OH, and OOH reaction intermediates and the scaling relationship between them. In general, the interlayer confinement destabilizes the adsorption of intermediates for the OER, but clustering Co atoms can selectively stabilize the adsorption of OOH in particular. After considering both effects, our model predicts an overpotential of 0.30 V for the Co-intercalated MnO2 catalyzed OER, in excellent agreement with the experimental result of 0.36 V. These new insights explain the enhanced catalytic performance of MnO2 by intercalating atoms and illuminate a route for engineering non-toxic precious-metal-free catalysts through designed layered materials. (C) 2019 Elsevier Inc. All rights reserved.