The electrocatalytic oxidative polymerization of o-phenylenediamine (o-PD) was performed on a reduced graphene oxide (RGO)/glassy carbon (GC) electrode. The electrolysis of o-PD was carried out using cyclic voltammetry and potentiostatic and galvanostatic methods. The experimental results demonstrated that the reduced graphene oxide (RGO) has a pronounced catalytic ability for the electrochemical oxidative polymerization of o-PD in a 0.60M H2SO4 solution compared to the bare GC electrode; however, graphene oxide has only a slight catalytic ability for the electrochemical oxidative polymerization of o-PD. The above three electrochemical techniques confirmed that there is a considerable discrepancy between the characteristics of the electrocatalytic oxidation of a species and the characteristics of the electrocatalytic oxidative polymerization of o-PD. This effect occurs because the charges passed during the electrolysis of o-PD on the bare GC electrode were mainly consumed for the formation of the soluble oligomer; however, RGO plays an important role in suppressing the formation of the soluble oligomer. An unexpected result was obtained: two or three pairs of the redox peaks of poly(o-phenylenediamine) (PoPD), synthesized using RGO as a catalyst, occur on the cyclic voltammogram in a wider potential range, depending on the polymerization conditions; however, only one pair of redox peaks occurs on the cyclic voltammogram of the conventional PoPD in a narrow potential range under exactly the same experimental conditions. The NMR and ESR spectra of the PoPD polymerized on the RGO/GC electrode are presented in this paper.