Deep geological repositories for the disposal of radioactive waste rely partly on the integrity of canisters and on the inhibition of microbial growth by the bentonite barrier for the effective isolation of the waste from the environment. Canister integrity can be compromised by the activity of sulfate-reducing bacteria (SRB) and by abiotic corrosion. Unexpected aerobic microbial growth and SRB inhibition under anoxic conditions were observed in bentonite during a recent long-term in-situ experiment, which raised the possibility that residual O₂ may delay anaerobic growth. Here, to investigate the role of O2, bentonite was equilibrated with 0, 21, or 100% O₂, compacted to 1.25 g/cm3, and deployed in a borehole for 1.5 years. Analyses revealed that the higher the O₂ concentration in bentonite, the greater the biomass and the more Desulfatitalea sp. dominates the SRB population. The thickest corrosion layer product of carbon steel was found in the 21% O₂ case, reflecting ongoing aerobic and anaerobic processes. In contrast, the most extensive structural Fe(III) reduction within montmorillonite was observed at 0% O₂. These findings demonstrate that residual bentonite O₂ shapes microbial activity and alters corrosion dynamics, highlighting the importance of accounting for oxygen during early repository evolution.