A fluorine-doped tin oxide-coated glass electrode modified with a bilayer film of underlying alpha-Co(OH)(2) and overlying Mg-intercalated and Co-doped delta-type (layered) MnO2 (Mg vertical bar Co-MnO2) preferentially yielded oxygen with a Faradaic efficiency as high as 79% in the presence of chloride ions at high concentration (0.5 M). This noble metal-free electrode was fabricated by cathodic electrolysis of aqueous Co(NO3)(2) followed by anodic electrolysis of a mixture of Mn2+, Co2+, and cetyltrimethylammonium (CTA(+)) ions in water. The CTA(+) ions accommodated in the interlayer spaces of Co-doped delta-MnO2, were replaced with Mg2+ by ion exchange. The upper MgCo-MnO2 could effectively block the permeation of Cl- ions and allow only H2O and O-2, while the under alpha-Co(OH)(2) acted as an oxidation catalyst for the H2O penetrated through the upper coating. Thus, the oxygen evolution reaction (OER) was preferred to the chlorine evolution reaction (CER). In artificial seawater (pH 8.3), the blocking effect against Cl- decreased because of ion exchange of the intercalated Mg2+ ions with Na+ in solution, but the OER efficiency still remained at 57%, much higher than that (28%) without the upper Mg vertical bar Co-MnO2. This demonstrates that the interlayer spaces between MnO2 layers acted as pathways for H2O molecules to reach the active sites of the underlying Co(OH)(2). Density functional theory (DFT) calculations revealed that the most stable structure of hydrated Mg2+ ion, in which a part of coordinated H2O molecules is hydrolyzed, has less affinity toward Cl- ion than that of hydrated Na+ ion.