Two energetic oxygen ion enhancements with a clear anti-sunward flow were observed in the dayside magnetosheath when GEOTAIL was located at (4.3, 16.3, 0.7) RE on 26 January 1994. The oxygen ions peaked at energies of about 300 keV. The commencement of the two oxygen enhancements at 1405 and 1440 UT were in coincidence with two substorm injections at 1349 and 1422UT. The oxygen events are characterized by strong impulsive increases in the flux whereas energetic protons and helium ions were essentially absent. The magnetosheath field projection in the x-y plane (GSE) was almost constant and pointed into the dusk/tail sector forming an angle of 135¿ to the Sun-Earth axis. The field magnitude was 35 nT with rather little fluctuations and the Bz component was negative. These observations show that energetic oxygen ions can leak out of the magnetosphere into the magnetosheath during isolated substorm activity (storm conditions are not required). Enhanced solar wind dynamic pressure compressed the magnetopause inward and may have helped to observe energetic oxygen ions to leave from the magnetopause current sheet thus escaping into the magnetosheath. ¿ 1999 American Geophysical Union