The thermalization of metastable oxygen atoms in the stratosphere and mesosphere has been investigated. Non-Maxwellian O(1D) distributions have been calculated at altitudes of 25 and 50 km taking into account the energy transfer and quenching collisions of fast O(1D) atoms with the ambient gas. The evolution of the energy distributions of nascent metastable oxygen atoms, produced by ozone photolysis, has been determined by solving the time-dependent Boltzmann equation. The time-dependent and steady state O(1D) distributions have been computed and used for calculations of parameters characterizing O(1D) thermalization and quenching in the stratosphere and mesosphere. The steady state O(1D) distributions contain 3--5% of nonthermal atoms, fractions which are significantly larger than those of nonthermal ground state oxygen or nitrogen atoms. The effective temperatures of the non-Maxwellian distributions have been found to be 14% and 33% higher than thermal temperatures of the ambient gas at 25 and 50 km. The nonequilibrium rate coefficients and yields of NO molecules in the atmospheric reaction O(1D) + N2O corresponding to the non-Maxwellian distributions of O(1D) atoms have been calculated.