The OH stretching vibration of jet-cooled phenol-N₂ in the neutral and cationic ground states was observed by using infrared-ultraviolet double resonance spectroscopy and infrared photodissociation spectroscopy, respectively. The OH vibration showed a small but significant low-frequency shift of 5 cm^-1 upon the cluster formation in the neutral, while the shift drastically increased up to 159 cm^-1 in the cation. These results represent the direct evidence of the in-plane cluster structure, in which phenolic OH is hydrogen bonded to N₂, as was proposed in the zero kinetic energy photoelectron study [S.R.Haines et al., J. Chem. Phys. 109, 9244 (1998)].