Nanostructured Co3O4-graphene hybrid catalysts are fabricated by a one-step vacuum kinetic spray technique from microparticles of Co3O4 and graphite powders. The Co3O4-graphene hybrid catalysts with various Co3O4 contents are studied concerning the oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) in 1.0 M KOH, as well as, H2O2 sensing in 0.1 M NaOH. We find that increasing graphene content in the hybrid catalysts results in an overall improvement of the OER electrocatalytic activity due to the enhancement in the charge transfer kinetics. The hybrid catalyst with 25 wt% Co3O4 reveals the optimum electrocatalytic activity toward the OER with the lowest overpotential (η) of 283 mV@ 10 mA cm−2 and superior reaction kinetics with a low Tafel slope of 25 mV.dec−1. Besides, the OER stability at 50 mA cm−2 for 50 h in 1.0 M KOH was verified. The hybrid catalyst with 50 wt% Co3O4 revealed the highest activity toward the HER with η of 108 mV@ 10 mA cm−2, Tafel slope of 90 mV.dec−1, and stability at 50 mA cm−2 for nearly 30 h. Moreover, it reveals ultrahigh H2O2 amperometric detection with superior sensitivity of 18,110 μA mM−1 cm−2, linear detection range from 20 μM to 1 mM, and a limit of detection of 0.14 μM.