As the simplest and most abundant dicarboxylic acid in the atmosphere, oxalic acid (OA) not only plays a key role in aerosol nucleation, but also acts as a prototypical compound for the investigation of intra- and intermolecular hydrogen-bonding interactions. A systematic theoretical study on the hydrated OA dimers performed by using DFT at the M06-2X/6-311++G(3df, 2p) level is discussed herein. The properties of hydrogen bonds in clusters are inspected through topological analysis by using atoms in molecules (AIM) theory. The most stable OA dimer involves a cyclic structure with two intermolecular hydrogen bonds. Calculations show that one H2 O has a slight effect on the hydrogen bonds, whereas two water molecules weaken and three water molecules break the two intermolecular hydrogen bonds between OAs. Furthermore, there are no hydrogen-bond interactions between OAs in almost all stable clusters as the number of H2 O molecules increases to four and five. Additionally, ionization and isomerization of OA through water-assisted proton-transfer phenomena are observed in tetra- and pentahydrates. This work provides new insights into the conversion of anhydrous OA into hydrated clusters that are helpful for further understanding the atmospheric nucleation process and nature of hydrogen bond.