Singly ionized atomic oxygen O+, was detected in the streaming plasmas within the erth's magnetotail at geocentric radial distances ~35 RE. These measurements were gained with a sensitive electrostatic analyzer on board the earth satellite Imp 7. Identification of the O+ ions was possible via distinct and persistent secondary maxima in the energy-per-unit-charge spectra of positive ions. Typical density ranges for H+ andO+ ions were 1--10 cm-3 and 10-3-10-2 cm-3, respectively. The plasma bulk flows were ~200--300 km s-1 and were directed generally tailward. Corresponding O+ fluxes in the magnetotail were ~105 (cm2 s)-1 and imply O+ upward fluxes in the upper ionosphere of ~108 (cm2 s)-1. The kinetic energy of the O+ ions is ~1--5 keV. Estimates of the global escape of terrestrial O+ ions are ~3¿106 kg yr-1.