On the basis of Geotail high energy particle-low energy particle detector (HEP-LD) observations, this paper reports on three energetic (144--4000 keV) oxygen burst sequences detected in the distant tail (Xgse=-40 to -66 RE) and their relation to substorm signatures. Those energetic oxygen ion bursts lasted only 20 to 30 min and exhibited strong beam-like structures. Two of the events (at about 1000 and 1900 UT on February 13, 1994) occurred in tailward flowing plasma after the flow direction changed from earthward to tailward; geostationary and ground based observations detected intense substorm activity during these periods, and the local magnetic field component Bz assumed predominantly negative values. The occurrence of a magnetic field with southward polarity and oxygen bursts embedded in tailward flowing plasma is consistent with the basic signatures of reconnection (formation of a neutral line) in the magnetotail. The third energetic oxygen burst with earthward flow was observed relatively close to Earth on August 27, 1993, X=-40 RE. No reversal in the plasma flow direction was seen, and the magnetic field polarity was essentially positive throughout the event. This is consistent with a near-Earth neutral line that had formed beyond X=-40 RE. We conclude that (1) a large amount of heavy ions from the ionosphere can be transferred to the distant tail and accelerated to high energies during substorm activity and that (2) these oxygen O+ ions from the polar ionosphere can be considered as tracer ions in the substorm dynamical process. ¿ 1998 American Geophysical Union