The O-H stretching Delta upsilon = 3, 4, and 5 vibrational overtone spectra of 2-butanol (sec-butyl alcohol) in the gas phase are measured using cavity ring-down spectroscopy (CRDS), where two bands are observed for each vibrational level. They are assigned to the absorption of the O-H stretching vibrations of different conformations and further confirmed by density functional theory (DFT) calculations. The mechanical frequencies omega(e), anharmonicities omega(e)chi(e), and dissociation energies D are evaluated by the local-mode theory. The band intensities of each band are obtained as well. The band shapes are simulated by a Lorentzian-Gaussian sum function. The molecular geometry, abundance, and O-H stretching frequencies of the calculated conformations are in good agreement with experimental results.