A combined experimental and theoretical study of the intermolecular vibrations of the o-xylene Ar van der Waals complex is reported for both the S-0 and S-1 electronic states. Two-color resonant two-photon ionization and fluorescence emission spectra of the vdW mode region of supersonic jet-cooled o-xylene Ar exhibit five bands within 70 cm(-1) of the electronic origin, which arise from low-frequency large-amplitude intermolecular vibrations. Accurate quantum 3D calculations of vdW vibrational levels were performed, based on the 3D discrete variable representation. Apart from the restriction to the J=0 state the calculated eigenstates are exact for the intermolecular potential energy surface (PES) employed. The PES is represented as a sum Lennard-Jones (LJ) pair potentials, and the direct comparison between theory and experiment enabled calibration of the LJ parameters. Very good agreement was achieved for both the S-0 and S-1 states of 0-xylene/Ar. The quantum 3D calculations provide a quantitative description of the vdW level structure up to approximate to 70 cm(-1) above the vdW ground state. The low-energy eigenfunctions have nodal patterns analogous to the 2,3-dimethylnaphthalene.Ar complex. However, in the energy range 40-60 cm(-1) the vdW mode eigenfunctions change over to 2D radial-oscillator-type wave functions, similar to those of benzene.Ar, but switch back to Cartesian type above 60 cm(-1). The S1 state vdW levels of 2,3-dimethylnaphthalene [M. Mandziuk, Z. Bacic, T. Droz, and S. Leutwyler, J. Chem. Phys. 100, 52 (1994)] were recalculated with the present parameters, and the agreement between experimental and calculated frequencies is improved.