Motivated by recent observations of highly variable hot plasma composition in the agnetosphere, control of the ionospheric escape flux composition by low-altitude particle dynamics and ion chemistry has been investigated for an e-, H+, O+ ionosphere. Theoretical polar wind results of Banks and Holzer and observations of highly nonthermal ion escape have been used as guides. It is found that the fraction of the steady state escape flux which is O+ can be controlled very sensitively by the occurrence of parallel or transverse ion acceleration at altitudes below the altitude where the neutral oxygen density falls rapidly below the neutral hydrogen density and the ionospheric source of O+ tends to be rapidly converted by charge exchange to H+. The acceleration is required both to overcome the gravitational confinement of O+ and to violate charge exchange equilibrium so that the neutral hydrogen atmosphere appears 'optically' thin to escaping O+. Constraints are placed on the acceleration processes, and it is shown that O+ escape is facilitated by observed ionospheric responses to magnetic activity.