In the same way that gases interact with oxide semiconductor surfaces from above, point defects interact from below. Previous experiments have described defect-surface reactions for TiO2(110), but an atomistic picture of the mechanism remains unknown. The present work employs computations by density functional theory of the thermodynamic stabilities of metastable states to elucidate possible reaction pathways for oxygen interstitial atoms at TiO2(110). The simulations uncover unexpected metastable states including dumbbell and split configurations in the surface plane that resemble analogous interstitial species in the deep bulk. Comparison of the energy landscapes involving neutral (unionized) and charged intermediates shows that the Fermi energy EF exerts a strong influence on the identity of the most likely pathway. The largest elementary-step thermodynamic barrier for interstitial injection trends mostly downward by 2.1 eV as EF increases between the valence and conduction band edges, while that for annihilation trends upward by 2.1 eV. Several charged intermediates become stabilized for most values of EF upon receiving conduction band electrons from TiO2, and the behavior of these species governs much of the overall energy landscape.