Characteristic features of the oxygen ion (O+) outflow in the nightside of Mars have been studied. The data set used includes all high telemetry O+ energy spectra observations made by the ASPERA particle instrument onboard the Phobos 2 spacecraft. Oxygen outflow within the optical shadow of Mars was detected on all orbits, but no clear structure was found when the ions were organised according to the cross-flow component of the interplanetary magnetic field (IMF). Oxygen ions in the tail outside the shadow were, in contrast, detected only on a few orbits and were related to fluctuating magnetosheath magnetic and electric fields with non-Parker spiral IMF orientation. The analysis indicates that O+ ions within the optical shadow from a ''steady'', ever persisting outflow feature. The high energy O+ ions in the tail outside the optical shadow are in contrast possibly a manifestation of the dynamic nature of the Martian tail, although effects associated to an asymmetric tail can not be fully excluded. ¿ American Geophysical Union 1995.