Abstract The replacement of noble metals with earth-abundant metals is still a big challenge for the practical application of electrocatalysis. In this work, we have developed the Mo x C-modified alloy@nitrogen-doped carbon hybrid electrocatalysts (Mo x C-alloy@NC, alloy: FeCo, NiCo) for oxygen evolution reaction (OER) by a simple thermolysis method. Compared with FeCo@NC and NiCo@NC, the OER performances of Mo x C-FeCo@NC and MoC-NiCo@NC are greatly enhanced, mainly due to the improved electrical conductivity by the introduce of Mo x C. Moreover, Mo x C-FeCo@NC exhibits a smaller Tafel slope (80 mV/dec) and a lower overpotential (318 mV) at 10 mA cm−2 in 1 M KOH solution, compared with MoC-NiCo@NC (186 mV/dec, 352 mV). In consideration of a lower BET area (6.6 m2 g−1) of Mo x C-FeCo@NC than those of MoC-NiCo@NC (25.4 m2 g−2), the remarkable electrocatalytic activity of Mo x C-FeCo@NC is mainly attributed to the presence of Mo(II) acting as the OER active species. Although Mo as hydrogen evolution reaction (HER) active species is well known, Mo(II) as the OER active species has not been reported before. Graphical abstract The introduction of Mo x C can not only improve the electrical conductivity, but also provide the Mo(II) active center, thus greatly enhanceing the OER activity of Mo x C-alloy@NC. This is the first finding that Mo(II) can act as the OER active species, which has not been reported before. Image 1 Highlights • Mo x C-alloy@NC electrocatalysts are prepared by a simple thermolysis method. • Mo x C significantly improve the electrical conductivity. • Mo(II) active species greatly increases the oxygen evolution reaction (OER) performance. • It is the first finding that Mo(II) can act as the OER active species. [ABSTRACT FROM AUTHOR]