The defect energy associated with oxygen-atom interstitials in magnesium oxide has been calculated using the Hartree-Fock (HF) Linear Combination of Atomic Orbitals (LCAO) method and the supercell (or large unit cell LUC) model. A neutral oxygen-atom interstitial was added at volume centred (v), face-centered (f), or edge-centered (e) position. The largest supercell used contained 32 lattice atoms, previous studies using the CNDO method having demonstrated the satisfactory convergence of defect energy, charge-distribution, and defect levels in the band gap, for supercells of this size. In all three defect positions (v, f, e) the most stable defect configuration is the symmetrical O^- - O^- 'dumbbell' formed by the displacement of one of the nearest-neighbour lattice ions, and accompanied by redistribution of charge such that the two O atoms in the dumbbell each have a charge close to -1e. The most stable defect position is the f dumbbell.