Rutile iridium dioxide (IrO) is renowned as an effective catalyst for the oxygen evolution reaction (OER), yet how its atomic structure governs orientation-dependent activity remains unclear. Here, we investigate epitaxial rutile IrO thin films with crystallographically well-defined (110), (101), and (001) orientations and show that their OER activity follows the order: (110) > (101) > (001). Atomic-resolution scanning transmission electron microscopy (STEM) analysis identifies distinct surface terminations and Ir-O coordination motifs that differ from commonly assumed idealized surface models. Experimentally informed electronic-structure calculations reveal that the (110) and (101) surfaces exhibit enhanced O 2p states near the Fermi level, favoring oxygen-involved charge transfer, whereas the (001) surface shows a more Ir-centered electronic character. Reaction-barrier calculations further indicate an energetic preference for the quadruple oxygen coupling mechanism (QOCM) on the (110) and (101) surfaces, whereas the (001) surface is more consistent with an Ir-centered adsorbate evolution mechanism (AEM)-like pathway. These results establish surface coordination as a key descriptor linking atomic termination, electronic structure, reaction pathway, and OER activity, providing a basis for designing high-performance IrO catalysts through surface-structure engineering.