Abstract: The terrestrial ring current, flowing near the equator at the altitude of 2 R_E～7 R_E, is one of the most significant electric current systems of the Earth's magnetosphere. Energetic ions (~1 keV to hundreds of keV), such as protons and oxygen ions, are thought to be the main carriers of the ring current. The enhancement of ring current during geomagnetic storms is widely regarded as the main reason for the geomagnetic field disturbance. After the storm main phase, it usually takes several days (i.e., storm recovery phase) for the ring current to return to the original level of quiet times. In this paper, we present some recent studies concerning the relative contributions of different particle species, especially oxygen ions, to the ring current and the loss mechanism of the ring current during the magnetic storm recovery phase. Partial pressures of different species increase significantly at high auroral electrojet levels with hydrogen pressure being dominant inside the plasmasphere. The pressures of the heavy ions and electrons increase outside the plasmapause and develop a strong dawn-dusk asymmetry with ion pressures peaking at dusk and electron pressures peaking at dawn. In addition, ring current hydrogen with energies ranging from 50 keV up to several hundred keV is the dominant component of plasma pressure during both quiet and active times, while helium contribution is generally small. Oxygen with 10 keV < E < 50 keV and electrons with 0.1 keV < E < 40 keV become increasingly important during active times contributing more than 25% and 20% on the nightside, respectively. O⁺ ions' contribution to ring current is strongly connected geomagnetic activity. During large storms when sym-H<−60 nT, abundant O⁺ ions are always present without exception and the relative pressure contribution can be comparable to that of H⁺ ions (ratio between O⁺ ions pressure and H⁺ ions pressure >0.8) and occasionally even larger than 1 when L < 3. Compared with the situation without much O⁺ ions contributing to the ring current, the O⁺ pressure and total plasma pressure are much larger when O⁺ ions are involved and the pressure peak moves to low L shells as sym-H decreases. Besides, the probability without much O⁺ is zero at most L shells when sym-H is less than −60 nT. These observational features suggest that O⁺ ions play an important role in the ring current during geomagnetic active times and no storm events exist without O⁺ ions being present. On the other hand, during relatively quiet conditions, the higher the probability, the lower of the value R. The strong correlation indicates that O⁺ ions are always absent during quiet times. During storm recovery phase, the observed lifetimes of H⁺ and O⁺ ions in general increase with L shell and the lifetimes of H⁺ ions are shorter than that of O⁺ ions when E < ~50 keV while the situation is reversed when E > ~50 keV, which is consistent with theory prediction. Furthermore, the observed lifetimes are in general consistent with model predictions of charge exchange lifetime, which confirms that charge exchange is a dominant loss mechanism of ring current ions during storm recovery phase.