A new 3d-5d hybridization oxide, CaCu3Mn2Os2O12 (CCMOO), was prepared by high-pressure and high-temperature synthesis methods. The compound crystallizes to an A-site-ordered but B-site-disordered quadruple perovskite structure with a space group of Im3̅ (No. 204). The charge states of the transition metals are determined to be Cu2+/Mn3.5+/Os4.5+ by X-ray absorption spectroscopy. Although most B-site-disordered perovskites possess lower spin-ordering temperatures or even nonmagnetic transitions, the current CCMOO displays a long-range ferrimagnetic phase transition with a critical temperature as high as ∼280 K. Moreover, a large saturated magnetic moment is found to occur [7.8 μB/formula units (f.u.) at 2 K]. X-ray magnetic circular dichroism shows a Cu2+(↑)Mn3.5+(↑)Os4.5+(↓) ferrimagnetic coupling. The corner-sharing Mn/OsO6 octahedra with mixed Mn and Os charge states make the compound metallic in electrical transport, in agreement with a specific heat fitting at low temperature. This work provides a rare example with high spin-ordering temperature and a large magnetic moment in B-site-disordered 3d-5d hybridization perovskite oxides.