We report a solid-state O-17 NMR study of the O-17 electric field gradient (EFG) and chemical shielding (CS) tensors for the oxonium ion, H3O+, in p-toluenesulfonic acid monohydrate (TAM). Both the O-17 EFG and CS tensors of the H3O+ ion are axially symmetric within the experimental errors. The O-17 quadrupole coupling constant (QCC) is found to be 7.05 +/- 0.02 MHz, and the O-17 chemical shift anisotropy (CSA)is 87 +/- 5 ppm. Experimental results are compared with extensive quantum chemical calculations using restricted Hartree-Fock approach (RHF), second-order Moller-Plesset perturbation theory (MP2), and density functional theory (DFT). The calculations showed that the strong hydrogen-bonding environment around the H3O+ ion in TAM is responsible for a reduction of approximately 3 MHz in the O-17 QCC compared to that of an isolated H3O+ ion. The effective O-17 quadrupole moment is calibrated at the B3LYP/cc-pVTZ level, Q = -2.400 fm(2). Using this value, we obtained the best calculated O-17 QCC for the "bound" H3O+ ion, +7.382 MHz, which is in reasonably good agreement with the observed value. The O-17 chemical shielding tensor is also calculated using the GIAO (gauge-including atomic orbital) approach. Although the calculated isotropic O-17 chemical shifts are in excellent agreement with the experimental data, the calculations with all the basis sets employed in the present study invariably underestimated O-17 CSAs by approximately 20 ppm.