Iridium-oxide-based catalysts are among the most active and stable materials for the anodic oxygen evolution reaction (OER) in acidic media, but even their longevity represents an important issue. It was recently demonstrated for many transition-metal oxides that stability of a catalyst can suffer from the active participation of lattice oxygen atoms in the OER. In this work, we combine density functional theory-based thermodynamics and molecular dynamics to analyze the OER activity of a series of Ir-bearing oxides. We reveal that although some Ir oxides exhibit thermodynamic overpotentials lower than that of the state-of-the-art IrO₂ rutile for the conventional reaction pathway, they also feature concomitant activation of lattice oxygen atoms toward the OER. By focusing on a few representative cases we unequivocally demonstrate that the lattice oxygen mechanism can outperform the conventional mechanism owing to its lower kinetic barriers. As lattice oxygen evolution was experimentally correlated with oxide degradation, enhanced OER activity due to involvement of lattice oxygen should compromise materials stability. This study highlights the importance of considering the lattice oxygen evolution reaction for more reliable computational predictions of electrochemically stable OER catalysts.