The decay of single and double quantum coherences in amorphous ortho-terphenyl is studied by means of deuteron NMR echo experiments near and below the calorimetric glass transition temperature T-g. In perdeuterated samples a Gaussian decay for these coherences is found at low temperatures, in accord with theoretical predictions for random local dipolar fields. A method is proposed to extract information about slow motional processes from ordinary solid echo experiments below T-g. Partly deuterated ortho-terphenyl does not show a Gaussian decay. Simple model calculations show that this observation indicates the importance of correlations of the local dipolar fields in the presence of protons.