Ruthenium/osmium based controlled size nanoparticles (NPs), supported on graphene, have been prepared by a simple synthetic strategy consisting in the thermolysis of a suitable precursor in organic solvent under a low oxygen enriched atmosphere. An excellent combination of a low Tafel slope of 28 mV/dec with a negligible overpotential was measured for the prepared nanocomposite (RuOs_G), due to metal oxides resulting in electron rich Ru and Os islands. The nanocatalyst shows an activity higher than that of common Pt. Additionally, the paper proves that it is possible to use the synthesized nanocatalyst to catalyse directly seawater splitting. RuOs_G resulted very stable and active in complex solutions during electrolysis under high current density.