Postsynthetic ion exchange of [Co₂(μ-Cl)₂(btta)] (MAF-X27-Cl, H₂bbta =1H,5H-benzo(1,2-d:4,5-d′)bistriazole) possessing open metal sites on its pore surface yields a material [Co₂(μ-OH)₂(bbta)] (MAF-X27-OH) functionalized by both open metal sites and hydroxide ligands, giving drastically improved electrocatalytic activities for the oxygen evolution reaction (an overpotential of 292 mV at 10.0 mA cm^-2 in 1.0 M KOH solution). Isotope tracing experiments further confirm that the hydroxide ligands are involved in the OER process to provide a low-energy intraframework coupling pathway.