Vibrational modes of substitutional oxygen on the anion site in cubic CdTe and wurtzite CdSe are identified from infrared (IR) absorption measurements on single crystals codoped with ¹⁸O and ¹⁶O. The ¹⁶O isotope variety of the defect in CdTe gives rise to a local vibrational mode at 349.8 (Formula presented.) (10 K), whereas the corresponding species in CdSe results in a doublet structure at 370.6 and 372.5 (Formula presented.) (10 K). The latter two modes are fully polarized for light parallel and perpendicular to the c axis of the CdSe crystal, respectively, and belong to the nondegenerate (Formula presented.) and the twofold degenerate E representations of the C3v point group.