The hypersonic expansion of O-2 through a nozzle is considered. The steady nonequilibrium vibrational distribution function of O-2, and the nonequilibrium forward reaction rate coefficient for the dissociation of O-2 are calculated theoretically. In the first instance, the vibrational relaxation of O-2 in the absence of reaction is examined in the temperature range 500-2500 K. The master equation for the vibrational populations, coupled to the steady one-dimensional conservation equations is solved numerically. The vibrational population obtained in this way, is compared to the distribution calculated using either a Treanor model or a Boltzmann distribution characterized by a vibrational temperature. The transition probabilities between O-2 vibrational levels employed take into account the vibrational anharmonicity, and the anisotropic intermolecular potential. For the temperature range 2500-5000 K, the vibrational relaxation including dissociation and recombination is studied. The reactive probabilities that are required for this calculation were obtained from the model of Kiefer and Hajduk [Chem. Phys. 38, 329 (1979)].