Irreversible parasitic reactions and the resulting accumulation of insulating side products are main barriers for practical application of rechargeable lithium-oxygen (Li-O-2) batteries. Therefore, it is critical to develop multifunctional oxygen electrodes suitable for oxygen reduction reaction, oxygen evolution reaction, and decomposition of side reaction products. Here we report the application of ultrafine ruthenium on boron carbide (Ru/B4C) with highly efficient multifunctional activities as carbon-free oxygen electrodes for Li-O-2 batteries. Li2CO3 and LiOH can be completely decomposed by Ru/B4C at 4.0 V and 4.1 V, respectively, within the stability window of the electrolyte. A Li-O-2 battery using the Ru/B4C oxygen electrode achieves low overpotentials for Li-O-2 reactions, and excellent cycling performance under the capacities of 300 and 500 mAh g(Ru/B4c)(-1). In-situ gas chromatography analysis reveals that O-2 is the major gas product during charging. Only a negligible amount of CO2 is observed in the first charging process. Therefore, Ru/B4C can be a very promising oxygen-electrode material for Li-O-2 batteries and Li-air batteries operated in ambient air.