The infrared spectroscopic properties of selected defects in forsterite are investigated by first-principles calculations, starting from previously proposed OH-incorporation mechanisms. The considered defects include the fully protonated M1 vacancy, fully protonated Si vacancy (hydrogarnet defect), and protonated Si vacancy associated with Ti incorporation at the M1 site (“titanoclinohumite” defect). Our findings establish that the previously proposed models for Mg and Ti defects are consistent with the observed infrared spectra. In contrast, a new model of Si defect is proposed, based on relative energies, vibrational frequencies, polarization properties and absorption coefficients. The theoretical results emphasize the critical role of the coupled vibration of OH groups in the relative intensities of OH stretching absorption bands. In contrast integrated absorption coefficients are less affected by coupling. The observed theoretical relation between the integrated absorption coefficient of OH defects and vibrational frequencies is consistent with recent experimental data.