The magnetic ground state in the double perovskite system Sr2−xCaxCoOsO6 changes from an antiferromagnet (x=0), to a spin glass (x=1), to a ferrimagnet (x=2) as the Ca content increases. This crossover is driven by chemical pressure effects that control the relative strength of magnetic exchange interactions. The synthesis, crystal structure, and magnetism of SrCaCoOsO6 and Ca2CoOsO6 are investigated and compared with Sr2CoOsO6. Both compounds adopt a monoclinic crystal structure with rock-salt ordering of Co2+ and Os6+ and a−a−b+ octahedral tilting, but the average Co–O–Os bond angle evolves from 158.0(3)∘ in SrCaCoOsO6 to 150.54(9)∘ in Ca2CoOsO6 as the smaller Ca2+ ion replaces Sr2+. While this change may seem minor, it has a profound effect on the magnetism, changing the magnetic ground state from antiferromagnetic in Sr2CoOsO6 (TN1=108K, TN2=70K), to a spin glass in SrCaCoOsO6 (Tf1=32K, Tf2=13K), to ferrimagnetic in Ca2CoOsO6 (TC=145K). In the first two compounds the observation of two transitions is consistent with weak coupling between the Co and Os sublattices.