The behavior of gold electrodes for the detection of dissolved oxygen was studied by the method of cyclic voltammetry in a phosphate-buffered solution with physiological pH. Surface modification with electropolymerized poly (o-phenylenediamine) film was performed to improve electrode antifouling properties. The voltammetric signature of oxygen was considered in terms of film electropolymerization conditions and post-deposition conditioning of the electrodes. The changes in the chemical structure of the poly (o-phenylenediamine) films as a result of these factors were confirmed by X-ray photoelectron spectroscopy analysis. Following long post-deposition conditioning in a phosphate-buffered solution, the modified electrodes exhibited stable voltammetric signatures in repeated tests and during storage as well as in the presence of a dense population of Escherichia coli (characterized by negligible metabolic activity) in the buffer. The results are indicative of the improved electrode antifouling properties.