Herein, a thermal decomposition method was utilised to fabricate pure and mixed manganese and ruthenium oxides as catalysts for the oxygen evolution reaction (OER). X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) reveal the manganese and ruthenium species produced at an annealing temperature of 600 °C to be Mn2O3 and RuO2/Ru, respectively. A number of the mixed Mn/Ru oxides exhibit overpotential values approximately 200 mV lower than previously reported for Mn2O3/RuO2 oxides (at a current density of 10 mA cm−2) for the OER, whereas the Mn 50 material exhibits similar overpotentials reported in the literature for RuO2. Turnover frequency (TOF) numbers for the Mn/Ru oxides were also calculated, and the results show that the TOF values for some of the materials in this work are higher than RuO2. XPS analysis indicates a change in chemical environment of the Mn/Ru materials, which exhibit higher TOF values. Subsequently, the pure Ru 100 material used in this study has a lower overpotential at 10 mA cm−2, as compared to previously reported values in the literature for RuO2 in alkaline media. This may be attributed to the presence of metallic Ru found in the film or the decrease in crystallite size, as determined by XRD. XPS analysis was also carried out after the OER to help determine the order of activity of the materials in this work.