The atomic and electronic structure of the radiation-induced Frenkel defects O i 0 in MgO crystals is calculated. A full-potential linear-muffin-tin-orbital method combined with a 16-atom supercell is used for the optimization of the interstitial defect geometry. It is found that energetically the most favorable ground state O i configuration is the ͑111͒ dumbbell centered at a regular oxygen site, whereas face-centered and cubecentered configurations are higher in energy by 1.45 eV and 3.57

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... respectively. The ͑111͒ configuration is close in energy to the ͑110͒ configuration, which allows the dumbbell to rotate easily on a lattice site. In all these four cases the interstitial oxygen atom attracts considerable additional electron density from its nearest regular O 2Ϫ ions, which makes it close to the O i Ϫ ion rather than a neutral atom. The mechanism and the relevant activation energy for O i diffusion are discussed in the light of available experimental data.