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The non-equilibrium dissociation of oxygen in air is studied using a quasi-one-dimensional model for hypersonic flow at a wide range of flight speed and altitude.The model solves the flow properties along the stagnation streamline using a quasi-one-dimensional formulation, and is found to be highly accurate for flow over blunt bodies as confirmed by computational fluid dynamics simulations.Its accuracy and efficiency allows to study the overall behavior of non-equilibrium dissociation during hypersonic flight.It is found that the maximum degree of oxygen dissociation increases at first and decreases thereafter with the altitude due to the competence between the shift of chemical equilibrium and non-equilibrium dissociation.The obtained oxygen dissociation pattern in the speed and altitude coordinates corrects the widely accepted equilibrium result of Hansen & Heims (1958, NACA-TN-4359).