We investigate the role of the O-vacancy in ZnO. The vacancy is well-known to be rich in ZnO and the origin of the natural n-type conductivity. The electronic structure of the O-vacancy examined by the accurate GW calculations indicates that it is a deep donor. By considering the interactions between the O-vacancy and the other native point defects in O-deficient ZnO through density-functional theory calculations, the presence of O-vacancy induces the formation of Zn-interstitial which is a shallow donor. A quantum mechanical attractive interaction between the deep O-vacancy donor and the shallow Zn-interstitial donor, which comes from the electronic orbital hybridization, is discussed. (C) 2011 Elsevier B.V. All rights reserved.