  Efficient electro-catalytic water-splitting technologies require suitable
catalysts for the oxygen evolution reaction (OER). The development of novel
catalysts could benefit from the achievement of a complete understanding of the
reaction mechanism on iridium oxide (IrO$_2$), an active catalyst material that
is, however, too scarce for large-scale applications. Considerable insight has
already been provided by \emph{operando} X-ray absorption near-edge structure
(XANES) experiments, which paved the way towards an atomistic description of
the catalyst's evolution in a working environment. We combine here
first-principles simulations augmented with a continuum description of the
solvent and electrolyte to investigate the electrochemical stability of various
IrO$_2$ interfaces and to predict the XANES cross-section for selected
terminations under realistic conditions of applied potential. The comparison of
computed O K-edge XANES spectra to corresponding experiments supports the
formation of electron-deficient surface oxygen species in the OER-relevant
voltage regime. Furthermore, surface hydroxyl groups that are found to be
stable up to $\sim$1 V are suggested to be progressively oxidized at larger
potentials, giving rise to a shift in the Ir L$_3$-edge cross-section that
qualitatively agrees with measurements.
