Although neutral in situ leaching through CO2 + O2 is employed to extract uranium (U) in sandstone by in situ leaching (ISL), mechanisms of U mobilization and O2 consumption remained unclear. To address this gap, 18 groundwater samples were taken from the Qianjiadian sandstone U ore field, including seven samples from production wells in mining area M1 (mining for 5 years), six samples from production wells in mining area M2 (mining for 4 years), and five samples from monitoring wells (GC), to quantify U‐mobilizing processes in the mining aquifer by employing hydrogeochemical compositions and multi‐isotopes. The introduction of O2 and CO2 efficiently stimulated U mobilization in the mining aquifer. The injected CO2 critically promoted the dissolution of carbonate minerals, which enhanced the formation of uranyl carbonate (predominantly CaUO2(CO3)22− and Ca2UO2(CO3)3(aq)) and thus facilitated U mobility. Generally, δ34SSO4 and δ18OSO4 in M2 and M1 were significantly lower than those in GC (p < 0.01). A Bayesian isotope mixing model of δ34SSO4 and δ18OSO4 showed that the contribution of pyrite oxidation to SO42− concentration increased from 1.7% in GC to 13.6% in M2 and to 15.0% in M1. During ISL, pyrite, ammonium, and dissolved organic carbon were major compounds competing with U(IV) for introduced O2 in the ore‐bearing aquifer. Most of the consumed O2 was used for pyrite oxidation (56.2%) and U(IV) oxidation (39.3%), following the thermodynamic sequence of those redox reactions. The current results highlighted the significance of increasing O2 utilization efficiency in improving the performance of ISL operations.