Polycrystalline Sr3OsO6, which is an ordered double-perovskite insulator, is synthesized via solid-state reaction under high-temperature and high-pressure conditions of 1200 degrees C and 6 GPa. The synthesis enables us to conduct a comparative study of the bulk form of Sr3OsO6 toward revealing the driving mechanism of 1000 K ferromagnetism, which has recently been discovered for epitaxially grown Sr3OsO6 films. Unlike the film, the bulk is dominated by antiferromagnetism rather than ferromagnetism. Therefore, robust ferromagnetic order appears only when Sr3OsO6 is under the influence of interfaces. A specific heat capacity of 39.6(9) X 10(-3) J mol(-1) K-2 is found at low temperatures (<17 K). This value is remarkably high, suggesting the presence of possible Fermionic-like excitations at the magnetic ground state. Although the bulk and film forms of Sr(3)OsO(6 )share the same lattice basis and electrically insulating state, the magnetism is entirely different between them.