We applied autoionization-detected infrared (ADIR) spectroscopy in order to observe OH stretching vibrations of jet-cooled 1- and 2-naphthol cations. In this technique, high Rydberg states, the vibrational levels of which are essentially the same as those of the corresponding bare molecular ion, were prepared by two-color double-resonance excitation. Vibrational transitions in the ion core of the high Rydberg states were measured by detecting the vibrational autoionization signal. For rotational and structural isomers of naphthol, similar low-frequency shifts of the OH frequencies upon ionization were found. The OH frequency shifts of naphthols were much smaller than that found for phenol, although both molecules have quite similar OH frequencies in their neutral ground state. This remarkable difference in frequency shifts was qualitatively explained in terms of the charge delocalization in the aromatic ring. In addition, the OH stretching vibrations of the 1- and 2-naphthol-Ar cluster cations were observed by infrared photodissociation spectroscopy. It was found that perturbations from the Ar atom to the hydroxyl group of naphthol are negligible in both the neutral and cationic ground states.