We investigated the properties of heavily oxygen-doped single GaN crystals and proposed their electronic structure based on first-principle calculations. The crystals were grown by carbothermal reduction and nitridation of Ga2O3 at 1180 degrees C for 2-8 h. The crystal were found to have high-crystallinity and high-concentration oxygen (4-9 x 10(20) atoms/cm(3)). Cathodoluminescence measurements of the 8-h-grown crystals showed a shift of band-edge-related luminescence peak toward higher energy (ca. 120 meV). Density of state of oxygen-doped GaN supercell was calculated using the first-principle to support this upward shift of luminescence energy. (C) 2007 Elsevier B.V. All rights reserved.