Elastic waves in the o-terphenyl single crystal belonging to the orthorhombic system were measured by the Brillouin light-scattering spectroscopy. Wave vectors of elastic waves were obtained from three principal values of dielectric tensor determined by a new simple method. All nine elastic constants representing elastic contribution to the free energy have been determined, which show intermediate anisotropy between benzene and linear polyphenyls. The anisotropy can reasonably be explained from the arrangement of the phenyl bonds and the inter-molecular interactions caused from the packing of molecules in the crystalline lattice. It has been found that pure transverse elastic waves would not be observed in this system by the Brillouin scattering.