Motivated by the recent report on the high-Tc ferromagnetic insulating state of a single transition metal containing a double perovskite compound, Sr3OsO6 [Wakabayashi et al., Nat Commun. 10, 535 (2019)2041-172310.1038/s41467-019-08440-6], we study this curious behavior by employing first-principles calculations in conjunction with exact diagonalization of the full t2g multiplet problem of two Os sites. Our analysis highlights the fact that stabilization of Sr3OsO6 in the cubic phase in epitaxially grown thin film is the key to both ferromagnetic correlation and the high-temperature scale associated with it. It also provides a natural explanation for why the sister compound, Ca3OsO6, exhibits low-TN antiferromagnetism in its monoclinic structure. Furthermore, the insulating property is found to be driven by the opening of a Mott gap in the half-filled spin-orbit coupled j=3/2 manifold of d2 Os. We point out that Sr2CaOsO6, which naturally forms in the cubic phase, would be worthwhile to explore as a future candidate to exhibit a high-Tc ferromagnetic insulating state in bulk form.