A knowledge of adsorption behaviors of oxygen on the model system of the reduced rutile TiO2(110)-1 x 1 surface is of great importance for an atomistic understanding of many chemical processes. We present a scanning tunneling microcopy (STM) study on the adsorption of molecular oxygen either at the bridge-bonded oxygen vacancies (BBOV) or at the hydroxyls (OH) on the TiO2(110)-1 x 1 surface. Using an in situ O-2 dosing method, we are able to directly verify the exact adsorption sites and the dynamic behaviors of molecular O-2. Our experiments provide direct evidence that an O-2 molecule can intrinsically adsorb at both the BBOV and the OH sites. It has been identified that, at a low coverage of O-2, the singly adsorbed molecular O-2 at BBOV can be dissociated through an intermediate state as driven by the STM tip. However, singly adsorbed molecular O-2 at OH can survive from such a tip-induced effect, which implies that the singly adsorbed O-2 at OH is more stable than that at BBOV. It is interesting to observe that when the BBO(V)s are fully filled with excess O-2 dosing, the adsorbed O-2 molecules at BBOV tend to be nondissociative even under a higher bias voltage of 2.2 V. Such a nondissociative behavior is most likely attributed to the presence of two or more O-2 molecules simultaneously adsorbed at a BBOV with a more stable configuration than singly adsorbed molecular O-2 at a BBOV.