In the present study, three mass-produced commercial IrOx samples from different suppliers were studied to establish correlations between various properties and their OER activities. The structures of the electrocatalysts at different scales were explored through laboratory instrumentation, powder X-ray diffraction, and synchrotron-based X-ray total scattering experiments combined with pair distribution function analysis. X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy using a transmission electron microscope were used to determine respectively the surface and the bulk elemental compositions of the samples. The coherent domain size (CDS) values of IrOx phases within the catalyst particles were estimated to be ∼10, ∼ 19, and ∼ 54 Å for the three IrOx samples. Surprisingly, the sample with a CDS of ∼19 Å turned out as the best OER electrocatalyst among the three in terms of mass-specific activity, IOER(m), followed by the 10 and 54 Å species. The amount of surface native compound oxygen was found to be a key parameter for the interface electrochemical accessibility. The intrinsic OER activity, evaluated using area-specific activity, IOER(a), suggests that the oxide with lattice disorder presenting a mixture of tetragonal and orthorhombic phases (70:20 w/w) is of superior intrinsic OER activity; however, the oxide with the presence of a monoclinic-like phase is of inferior intrinsic OER activity, which may also be due to the surface presence of Ir3+ along with Ir4+. The classic belief that the pure tetragonal phase is the best crystalline structure as the OER catalyst is challenged. Iridium oxides with disordered crystallinities may offer a class of highly active oxygen evolution electrocatalysts. The knowledge thus obtained should have a significant impact on the understanding, selection, and processing of IrOx-based OER electrocatalysts.