Formation of an electroactive composite based on graphene oxide and poly-o-phenylenediamine (PPD) was studied. Electron absorption spectra were used to confirm formation of a strong chemical bond at a prolonged contact of unreduced graphene oxide on a conducting support (glassy carbon, SnO2) and o-phenylenediamine monomer solution. Here, graphene oxide is partially reduced and oxidation and partial polymerization of o-phenylenediamine starts. It is shown that electrochemical oxidation of the obtained composite under the conditions of cyclic voltammetry results in the further polymerization of o-phenylenediamine bound to graphene oxide; here, reduction of graphene oxide continues, and at much lower cathodic potentials than in the absence of o-phenylenediamine. Morphology of the obtained composite was studied using the AFM technique. PPD embedded into the composite structure does not allow nanosheets of reduced graphene oxide (RGO) to corrugate and imparts the morphology of the composite the shape of globules with a clearly pronounced structure. As a result, the RGO–PPD composite was obtained that demonstrated pronounced electroactivity in a wider range of potentials than in the case of nonmodified PPD.