The utility of a primitive semiclassical method for the quantitative prediction of vibrational eigenvalues and electric dipole transition intensities in triatomic molecules is assessed for the particular case of rotationless OCS in its ground electronic state by comparison to exact quantum calculations. The semiclassical method is based on numerical integration of appropriately selected classical trajectories. The potential energy function determined by Foord, Smith, and Whiffen [Mol. Phys. 29, 1685 (1975)] and the electric dipole moment function determined by Tanaka, Tanaka, and Suzuki [J. Chem. Phys. 82, 2835 (1985)] provide the model of the OCS system. Eigenvalues are obtained by the method of adiabatic switching, the number of trajectories required for this purpose being minimized to four using an extension of Johnson’s Fourier series method [J. Chem. Phys. 83, 1204 (1985)]. The resulting semiclassical vibrational transition frequencies (with respect to the ground state) agree with the corresponding quantum frequencies to within 1 cm−1 (2 cm−1) for 112 (128) of the 145 converged quantum levels, with the largest discrepancy being 5.2 cm−1. The semiclassical frequencies are compared to the experimental results of Fayt [Ann. Soc. Sci. Brux. 86, 61 (1972)] and to the semiclassical results of Colwell [Chem. Phys. 46, 165 (1980)]. The sets of 176 semiclassical and 145 converged quantum transition frequencies reported here are the most extensive and complete to date, the highest energy level being ∼7500 cm−1 above the ground state. The methodology of Wardlaw, Noid, and Marcus [J. Phys. Chem. 88, 536 (1984)] for the determination of semiclassical transition intensities in 2D oscillator systems is herein extended to the vibrational degrees of freedom in triatomic molecules. For numerous transitions from the ground state and from several low-lying excited states, the semiclassical intensities agree with the quantum intensities to within 6% in the absence of resonances in the associated approximate eigentrajectories. When resonances are involved, the primitive semiclassical treatment is found to be far less accurate, as is expected. A numerical determination of the classical actions, Fourier spectra of the coordinates, surfaces of section, and 2D slices through configuration space are presented for representative resonant and nonresonant approximate eigentrajectories. The inherent uncertainties in the semiclassical energy levels and in the transition intensities (if no resonant trajectories are involved) are found to provide a very reliable upper bound on the difference between the semiclassical and quantum results.