*

We present the results of the terahertz (THz) conductivity studies in the double perovskite family of 5d-oxides Sr_2-xCa_xCoOsO₆. As a function of composition x, this family of double perovskites undergoes a transformation from robust antiferromagnetic order at x=0, to spin glasses at x=1, and then eventually to a ferrimagnetic state at x=2. We find that concomitant with these changes is the steady increase of the THz conductivity that cannot be explained with the expected changes to the optical phonon frequency with composition. We discuss the possibility of these changes being associated with the increase of magnetic disorder as a function of composition, particularly around x=1.