We present results of our theoretical study on the mechanism and thermodynamics of the incineration of oxalic acid H2C2O4 on a boron-doped diamond electrode. The reaction is assumed to occur in aqueous solution in several steps, of which the first, the most important, is the generation of the OH radical according to an outer sphere mechanism. The energy balance and barrier for this step as well as for all subsequent steps (OH reaction with oxalic acid leading to creation of HC2O4 radical, its C-C bond dissociation and OH reaction with the COOH radical) were evaluated by performing quantum calculations with the MP2 and three DFT methods and the aug-cc-pVTZ basis set. The results indicate that the first step is characterized by a rather low activation energy of 0.25 eV and by a relatively high transfer coefficient of similar to 0.7. The next process - detachment of H atom from oxalic acid by OH radical - meets an energy barrier of 0.55 eV; for all subsequent steps the activation energies are much smaller. The overall incineration reaction is found to be highly exothermic by ca. 7 eV. Our results are also applicable to similar inert electrodes.