Formic acid adsorption on rutile TiO2(110) single crystals exposed to different oxygen partial pressures have been investigated with infrared reflection-absorption spectroscopy (IRRAS) employing p- and s-polarized light incident along the  crystal direction at a temperatures between 273K and 303K. IRRAS spectra, prior to oxygen exposure, shows that formic acid dissociates upon adsorption and binds to the surface as bridging formate species with the symmetric νs(OCO) and asymmetric νas(OCO) peak at 1360 and 1531 cm-1, respectively. A new asymmetric νas(OCO) band at 1517 cm-1develops in the presence of O2, which is correlated with the O2 partial pressure, whereas no changes is seen in the symmetric νs(OCO) band. At low O2 pressures(<1 ×10-7mbar), this band is not detectable within 3 h of O2 exposure. At higher O2pressures, 5 ×10-7 mbar, the band starts to develop, and at 1 ×10-6 mbar, a pronounced band develops after 30 min exposure. The O2 induced band at 1517 cm-1 does not disappear as the O2 gas is removed, suggesting that a stable chemical modification of adsorbed formate molecules. No changes were observed in the s-polarized IRRAS spectra with O2 exposure, showing that there is no rotation of the formate molecules in the plane of the surface. The appearance of the new νas(OCO) band are consistent with a model where O2 dissociated to form O adatoms bonded to the Ti5c atoms, which interact with bridging formate molecules bonded along the  direction. The relationship between the oxygen partial pressure and the formation of this new band is due to the increased larger interaction between the surface and the oxygen with increase pressure or an increased oxygenation of formate. Our results provide new insight into the interaction between O2and TiO2surfaceat elevated pressures relevant for practical applications.