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Discovering highly active and stable electrocatalysts for the oxygen evolution reaction (OER) is critical to the commercial development of many next-generation energy conversion and storage devices, with Fe-doped nickel (oxy)­hydroxide representing one of the most promising OER catalysts developed to date. However, the active sites and mechanism of OER on Fe-doped nickel (oxy)­hydroxide catalysts remain unclear. To gain deeper insights into the role of metal dopants in enhancing OER activity, we explored here the role of Ir-doping in the OER performance of nickel (oxy)­hydroxide catalysts, placing particular emphasis on the nature of the active site. Density functional theory calculations with Hubbard U correction revealed that Ir-doping of a β-NiOOH(001) surface enhanced the electric conductivity while also activating an oxygen site involving three Ni atoms (Ni3 site) to realize a remarkably low OER overpotential of only η = 0.46 V, much lower than the overpotential on the oxygen site involving Ir + two Ni atoms (IrNi2 site, η = 0.77 V) or the oxygen site involving three Ni atoms in pristine β-NiOOH (η = 0.66 V). Guided by the computational results, ultrathin Ir-doped Ni­(OH)2 nanosheets were then fabricated through a combination of hydrothermal assembly and liquid exfoliation, with the nanosheets transforming to Ir-doped NiOOH during OER and offering superior activity relative to pristine Ni­(OH)2 nanosheets or a commercial IrO2 catalyst, thereby validating the theoretical predictions. The computational and experimental results thus conclusively demonstrate that Ir-doping and nanosheet engineering are synergistic strategies for tuning the electronic and structural properties of nickel (oxy)­hydroxides for improved oxygen evolution electrocatalysis.