Geometric and vibrational spectroscopic data (rotational constants, bond distances and angles, vibrational frequencies, IR intensities, and OH/OD isotope effects) of phenol, benzaldehyde, and salicylaldehyde as calculated at various levels of theory (HF/6-31G(d,p), HF/6-311++G(d,p), MP2/6-31G(d,p), B3P86/6-31G(d,p), BLYP/6-31G(d,p), B3LYP/6-31G(d,p), and B3LYP/6-311++G(d,p)) are reported. The theoretical results are discussed mainly in terms of comparisons with available experimental data. For geometric data (rotational constants and bond distances) the best agreement between theory and experiment is obtained at the MP2 and B3LYP levels. B3P86 calculated data are slightly worse, while HF and BLYP calculations yield distinctly too small and too large bond distances, respectively. B3LYP calculated vibrational spectroscopic data excellently agree with experimental IR spectra for phenol, phenol-OD, and benzaldehyde, and with minor restrictions, also for salicylaldehyde and salicylaldehyde-OD. Considering frequency sequences, IR intensities, and OH/OD isotope effects, reliable and consistent assignments are given. BLYP and B3P86 calculated vibrational spectroscopic data are slightly worse, whereas MP2 and HF calculations suffer from several shortcomings that are already known from calculations of smaller molecules, such as benzene.