Glyoxal (GLY) is an important precursor of aqueous secondary organic aerosol (aqSOA). Knowledge of the reaction mechanisms of GLY in the aqueous phase is not complete, and the oxidation mechanisms of GLY in the presence of different oxidants are under debate. Our recent experimental studies of GLY oxidation mediated by nitrate photolysis yielded formic acid instead of oxalic acid which was commonly found in bulk GLY oxidation experiments. In this work, we investigated the hydration and oligomer formation mechanism of GLY in the aqueous phase theoretically. Furthermore, we calculated the energetics for the mechanisms of GLY oxidation by OH radicals in the presence of O2 and NO2. Results showed that the hydration reaction plays a significant role in GLY uptake into the aqueous phase, and oligomers are likely to form at intermediate RH. The formation of formic acid in the presence of OH and NO2 is more favorable, while oxalic acid is the main product in the presence of OH and O2. The present study provides a theoretical insight into the aqueous-phase chemistry of GLY, and the results may be helpful for understanding the atmospheric evolution of GLY.