Chemical and electrochemical functionalization of graphene oxide by o-phenylenediamine with its subsequent polymerization was conducted. Electrochemical functionalization was carried out by anodic oxidation of o-phenylenediamine on the graphene oxide-covered gold electrode, while chemical functionalization was made by incubation of graphene oxide-covered electrode in the monomer solutions of different pHs. Cyclic voltammetry with the simultaneous mass changes monitored by EQCM was used for the testing of the resulting electrodes. The results demonstrate that formation of poly(o-phenylenediamine) takes place not only by electrochemical oxidation of monomer but also as a result of the spontaneous heterogeneous redox reaction between oxygen functionalities on graphene oxide and o-phenylenediamine. Resulting composite of graphene oxide/poly(o-phenylenediamine) showed relatively stable response and increased currents over the wide potential range in comparison with the same polymer formed at bare gold electrode giving rise to the significant pseudocapacitance of this material. The electroactivity of the composite was preserved in neutral medium, and hydration was identified as a rate-limiting process.