Boron doped diamond films (BDD) are the most studied electrocatalytic materials and their efficiency for removing organic compounds is attributed to their ability to produce a large amount of hydroxyl radicals by the electrolysis of water. However, some organic compounds are easy to degrade than others, as well as they also interact with BDD surface. Thus, it is necessary to understand the chemical/electrochemical process at the molecular level. In this frame, this work aims to study the electrochemical oxidation (EO) of oxalic acid (OA) at BDD anode, understanding the reaction mechanism, formation of hydroxyl radicals, the effect of supporting electrolyte concentration, and the role of oxidants in solution taking into consideration the cyclic and linear voltammetric measurements. Voltammetric results clearly reveal that a direct electron transfer is achieved as the main mechanistic behavior suffered by OA on BDD surface before oxygen reaction. However, the concentration of sulfates in solution plays an important role in the solvation, diffusion and adsorption species at BDD surface, promoting parallel mechanisms. From the information provided by experimental measurements, the preferential mechanism on BDD surface at low or high overpotential regions was elucidated.