Discovering precious metal-free electrocatalysts exhibiting high activity and stability toward both the oxygen reduction (ORR) and the oxygen evolution (OER) reactions remains one of the main challenges for the development of reversible oxygen electrodes in rechargeable metal–air batteries and reversible electrolyzer/fuel cell systems. Herein, a highly active OER catalyst, Fe0.3Ni0.7OX supported on oxygen-functionalized multi-walled carbon nanotubes, is substantially activated into a bifunctional ORR/OER catalyst by means of additional incorporation of MnOX. The carbon nanotube-supported trimetallic (Mn-Ni-Fe) oxide catalyst achieves remarkably low ORR and OER overpotentials with a low reversible ORR/OER overvoltage of only 0.73 V, as well as selective reduction of O2 predominantly to OH−. It is shown by means of rotating disk electrode and rotating ring disk electrode voltammetry that the combination of earth-abundant transition metal oxides leads to strong synergistic interactions modulating catalytic activity. The applicability of the prepared catalyst for reversible ORR/OER electrocatalysis is evaluated by means of a four-electrode configuration cell assembly comprising an integrated two-layer bifunctional ORR/OER electrode system with the individual layers dedicated for the ORR and the OER to prevent deactivation of the ORR activity as commonly observed in single-layer bifunctional ORR/OER electrodes after OER polarization.