Abstract It was recently discovered that in irradiated germanium doped with oxygen a local vibrational mode occurs at 669 cm −1 which could be ascribed to the negatively charged oxygen–vacancy (VO − ) complex. In the 669 cm −1 band and in another band at 731 cm −1 due to a different defect, fine structure which indicated the occurrence of the Ge-isotope splitting of the modes could be partly resolved. We report here the results of high-resolution infrared measurements (res.=0.02 cm −1 ) at liquid helium temperatures of the two bands. The oxygen-doped germanium ([O i ]=2.7×10 17 cm −3 ) was first irradiated with 2 MeV electrons to increase the concentration of VO complexes. A second similar specimen received the same treatment but with subsequent anneals to obtain the 731 cm −1 band. The experimentally observed splitting of the 669 cm −1 band and the amplitudes of its individual components are accurately predicted using a model of a non-linear symmetric Ge–O–Ge molecule within a Ge crystal lattice, in agreement with the assignment of the mode to the VO − complex. A similar model also seems to apply to the yet unassigned 731 cm −1 mode and the conclusion is made that the defect also must include a single O atom bonded to two equivalent Ge atoms. The higher annealing temperature of the unassigned band however indicates that the defect must be more complex than the VO center.