The O-H stretching Deltav = 3, 4, and 5 vibrational overtone spectra of isobutyl alcohol (isobutanol, 2-methyl-1-propanol) are measured by using cavity ring-down spectroscopy (CRDS), where three bonds are observed for each vibrational level. They are assigned to the O-H stretching vibrational absorption of the three kinds of conformations of the hydroxyl group and further confirmed by density function theory (DFT) calculations using highly accurate B3LYP/6-31+G(d,p) and 6-311+G(d,p) levels. The experimental 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 calculated O-H stretching Deltav = 1, 3, 3, and 5 spectra are simulated by Lorentzian-Gaussian sum function in order to fit the experimental spectra. The molecular vibrational abundance and O-H stretching frequencies of different vibrations of the calculated molecular conformations are in agreement with the experimental results.