Na2OsO8 crystallizes in the monoclinic P2_1 space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to one O(2), one O(4), one O(5), and one O(6) atom to form distorted NaO4 trigonal pyramids that share  a cornercorner with one Na(2)O5 trigonal bipyramid, corners with three equivalent Os(1)O5 trigonal bipyramids, and  an edgeedge with one Na(2)O5 trigonal bipyramid. In the second Na site, Na(2) is bonded to one O(1), one O(2), one O(3), one O(5), and one O(6) atom to form NaO5 trigonal bipyramids that share corners with three equivalent Os(1)O5 trigonal bipyramids,  a cornercorner with one Na(1)O4 trigonal pyramid, and  an edgeedge with one Na(1)O4 trigonal pyramid. Os(1) is bonded to one O(3), one O(4), one O(5), one O(6), and one O(8) atom to form OsO5 trigonal bipyramids that share corners with three equivalent Na(2)O5 trigonal bipyramids and corners with three equivalent Na(1)O4 trigonal pyramids. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a water-like geometry to one Na(2) and one O(7) atom. In the second O site, O(2) is bonded in a distorted bent 150 degrees geometry to one Na(1) and one Na(2) atom. In the third O site, O(3) is bonded in a bent 150 degrees geometry to one Na(2) and one Os(1) atom. In the fourth O site, O(4) is bonded in a distorted bent 120 degrees geometry to one Na(1) and one Os(1) atom. In the fifth O site, O(5) is bonded in a distorted T-shaped geometry to one Na(1), one Na(2), and one Os(1) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Na(1), one Na(2), and one Os(1) atom. In the seventh O site, O(7) is bonded in a single-bond geometry to one O(1) atom. In the eighth O site, O(8) is bonded in a single-bond geometry to one Os(1) atom.
Na2OsO8 crystallizes in the monoclinic P2_1 space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to one O(2), one O(4), one O(5), and one O(6) atom to form distorted NaO4 trigonal pyramids that share  a cornercorner with one Na(2)O5 trigonal bipyramid, corners with three equivalent Os(1)O5 trigonal bipyramids, and  an edgeedge with one Na(2)O5 trigonal bipyramid. The Na(1)-O(2) bond length is 2.39 Å. The Na(1)-O(4) bond length is 2.33 Å. The Na(1)-O(5) bond length is 2.50 Å. The Na(1)-O(6) bond length is 2.33 Å. In the second Na site, Na(2) is bonded to one O(1), one O(2), one O(3), one O(5), and one O(6) atom to form NaO5 trigonal bipyramids that share corners with three equivalent Os(1)O5 trigonal bipyramids,  a cornercorner with one Na(1)O4 trigonal pyramid, and  an edgeedge with one Na(1)O4 trigonal pyramid. The Na(2)-O(1) bond length is 2.79 Å. The Na(2)-O(2) bond length is 2.34 Å. The Na(2)-O(3) bond length is 2.40 Å. The Na(2)-O(5) bond length is 2.38 Å. The Na(2)-O(6) bond length is 2.35 Å. Os(1) is bonded to one O(3), one O(4), one O(5), one O(6), and one O(8) atom to form OsO5 trigonal bipyramids that share corners with three equivalent Na(2)O5 trigonal bipyramids and corners with three equivalent Na(1)O4 trigonal pyramids. The Os(1)-O(3) bond length is 1.78 Å. The Os(1)-O(4) bond length is 1.91 Å. The Os(1)-O(5) bond length is 1.79 Å. The Os(1)-O(6) bond length is 1.82 Å. The Os(1)-O(8) bond length is 1.76 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a water-like geometry to one Na(2) and one O(7) atom. The O(1)-O(7) bond length is 1.24 Å. In the second O site, O(2) is bonded in a distorted bent 150 degrees geometry to one Na(1) and one Na(2) atom. In the third O site, O(3) is bonded in a bent 150 degrees geometry to one Na(2) and one Os(1) atom. In the fourth O site, O(4) is bonded in a distorted bent 120 degrees geometry to one Na(1) and one Os(1) atom. In the fifth O site, O(5) is bonded in a distorted T-shaped geometry to one Na(1), one Na(2), and one Os(1) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Na(1), one Na(2), and one Os(1) atom. In the seventh O site, O(7) is bonded in a single-bond geometry to one O(1) atom. In the eighth O site, O(8) is bonded in a single-bond geometry to one Os(1) atom.