We propose a RIXS experiment to identify the magnetic interaction and electronic structure in the perovskite KOsO3 synthesized recently. KOsO3 has a simple cubic perovskite structure and remains magnetic up to 800 K. The refinement of neutron diffraction gives an incommensurate spin structure below TN. KOsO3 remains a good conductor to the lowest temperature. The unusual spin structure suggests that KOsO3 may be at the crossover from localized to itinerant electronic behaviour. RIXS has been used to identify correlation fluctuations in the metallic phase above TN in the orthorhombic NaOsO3, which is a key factor to attribute the puzzling metal-insulator transition to the Mott transition in the perovskite. This proposal aims to construct a microscopic picture of the electronic state at the crossover by studying a variety of electron/orbital excitations with RIXS probe.