The small or near-zero band gap of germanium found by supercell local density functional treatments causes difficulties in the study of the properties of charged defects in Ge. Here, we use large H-terminated Ge clusters together with a non-linear core corrected (NLCC) Ge pseudopotential to explore the structure, vibrational and electrical properties of the vacancy–oxygen complex (VO). The cluster possesses a gap sufficiently wide to contain defect related energy levels, thus allowing us to model the three charge states of the defect. The local vibrational modes (LVM) calculated for the neutral ( VO 0 ), negatively charged ( VO - ) and double negative ( VO = ) defect are 602, 684 and 694 cm - 1 , and can be favourably compared with experimental values of 621, 669 and 716 cm - 1 , respectively [Markevich VP, et al. Physica B 2003; 340–2, 844–8]. Using substitutional gold ( Au s ) as a marker defect, electric levels of VO are found at E ( -/ 0 ) = E v + 0.30 eV and E ( = /- ) = E c - 0.29 eV , in excellent agreement with the respective experimental enthalpies for hole and electron emission Δ H h ( -/ 0 ) ≃ 0.32 eV and Δ H e ( = /- ) = 0.26 eV , respectively. Finally, the migration, dissociation and reorientation energies of the defect are also reported.