The total and differential cross sections of elastic O(3P)+O(3P) collisions have been obtained from quantal calculations, and the rate of energy relaxation of fast oxygen atoms in atmospheric oxygen gas has been evaluated using realistic differential cross sections of oxygen-oxygen collisions. All the electronic energy curves of O2 separating to the atomic ground states have been incorporated in calculations of the scattering phase shifts. The total and differential cross sections averaged over a statistical population of O(3P) fine structure levels have been used for calculations of the self-diffusion coefficient, the viscosity and thermal conductivity of oxygen gas. The kernel of the Boltzmann equation for the energy relaxation process has been determined. The rate of energy losses of energetic oxygen atoms in the upper terrestrial atmosphere are calculated for typical thermospheric conditions. The energy distribution function of the recoil oxygen atoms is constructed using the computed energy relaxation kernel. ¿ 2000 American Geophysical Union