It has been observed that the vibrational stretching frequency of a C−H covalent bond commonly shifts to the blue and suffers intensity loss, when the CH engages in a hydrogen bond. However, the shift does not always occur in this direction, and there are cases when a CH blue shift may be present even in the absence of a CH···O interaction. Ab initio quantum calculations are used to analyze the structure, and vibrational and NMR spectra of small model systems containing both conventional and CH···O H-bonds, and thereby identify patterns that unambiguously signal the presence of a CH···O interaction.