The synthesis, structures and magnetism of six mixed 3d-5d oxides Ba3BM2O9 (B = Ti, Y, Zn; M = Ru, Os) are described. When prepared at ambient pressure the six oxides display a 6H type perovskite structure comprised of corner sharing BO6 and face sharing M2O9 motifs. Synchrotron X-ray diffraction reveals a small monoclinic distortion in Ba3ZnRu2O9; the remaining oxides exhibit a hexagonal structure. The magnetic properties are dominated by the M-M interactions across the shared face. Only in the mixed valent (M4+/M5+) Y oxides is evidence of long-range magnetic order found. Application of high pressure/high temperature synthetic methods for the Ru containing oxides changes the structure to the archetypical cubic Pm3[combining macron]m perovskite structure, where the B and Ru cations are disordered on the corner sharing BO6 octahedral sites. The magnetic properties of the cubic oxides are dominated by short range antiferromagnetic interactions, the chemical disorder inhibiting long range ordering.