Synthesis, crystal structure, and magnetic properties of a new 2:1 ordered triple perovskite Sr₃CaOs₂O₉ are reported. The compound crystallizes in a P2₁/c space group and features a unique buckled honeycomb lattice of osmium. It exhibits longrange antiferromagnetic ordering with a high Néel temperature of 385 K as confirmed by susceptibility, heat capacity, and neutron diffraction measurements and is electrically insulating. This compound is also the first example of a 2:1-ordered osmate perovskite. Theoretical investigations indicate that Sr₃CaOs₂O₉ features a sizeable antiferromagnetic exchange between the puckered planes resulting in a high T_N. The magnetic properties of the known compound Sr₃CaRu₂O₉ are elucidated in comparison. It shows antiferromagnetic order below T_N ≈ 200 K.