  Polycrystalline Sr$_3$OsO$_6$, which is an ordered double-perovskite
insulator, is synthesized via solid-state reaction under high-temperature and
high-pressure conditions of 1200 $^\circ$C and 6 GPa. The synthesis enables us
to conduct a comparative study of the bulk form of Sr$_3$OsO$_6$ toward
revealing the driving mechanism of 1000 K ferromagnetism, which has recently
been discovered for epitaxially grown Sr$_3$OsO$_6$ films. Unlike the film, the
bulk is dominated by antiferromagnetism rather than ferromagnetism. Therefore,
robust ferromagnetic order appears only when Sr$_3$OsO$_6$ is under the
influence of interfaces. A specific heat capacity of 39.6(9) 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.
