Solid-state O-17 NMR spectroscopy is employed to characterize powdered samples of known monoclinic and orthorhombic modifications of O-17-enriched triphenylphosphine oxide, Ph3PO. Precise data on the orientation-dependent O-17 electric field gradient (EFG) and chemical shift (CS) tensors are obtained for both polymorphs. While the O-17 nuclear quadrupolar coupling constants (C-Q) are essentially identical for the two polymorphs (C-Q = -4.59 +/- 0.01 MHz (orthorhombic); C-Q = -4.57 +/- 0.01 MHz (monoclinic)), the spans (Omega) of the CS tensors are distinctly different (Omega = 135 +/- 3 ppm (orthorhombic); Omega = 155 +/- 5 ppm (monoclinic)). The oxygen CS tensor is discussed in terms of Ramsey's theory and the electronic structure of the phosphorus-oxygen bond. The NMR results favor the hemipolar sigma-bonded R3P+-O- end of the resonance structure continuum over the multiple bond representation. Indirect nuclear spin-spin (J) coupling between P-31 and O-17 is observed directly in O-17 magic-angle-spinning (MAS) NMR spectra as well as in P-31 MAS NMR spectra. Ab initio and density-functional theory calculations of the O-17 EFG, CS, and (1)J(P-31,O-17) tensors have been performed with a variety of basis sets to complement the experimental data. This work describes an interesting spin system for which the CS, quadrupolar, J, and direct dipolar interactions all contribute significantly to the observed O-17 NMR spectra and demonstrates the wealth of information which is available from NMR studies of solid materials.