Functional groups are sets of atoms whose presence in a molecule confers to it chemical properties that are characteristic of the group.  The atoms  that compose the functional group  have a deformation pattern that can be easily recognized, with slight variations caused by the molecular environment. This is illustrated in the following pictures, where the accumulated deformation of atoms in the -COOH group of formic and benzoic acids and in the -OH group of methanol and phenol have been drawn for contour values of ±0.05 au.