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Quasielectrostatic instabilities driven by anisotropic oxygen ions are investigated during storms/substorms in three-component ring current plasma consisting of electrons, protons, and energetic oxygen ions having loss-cone distribution. The response of the electrons is fully electromagnetic and that of the protons and oxygen ions is electrostatic. The electrons, protons, and oxygen ions are treated as magnetized. Obliquely propagating modes with frequencies near the harmonics of the oxygen ion cyclotron frequency are found to become unstable due to pressure anisotropy of the energetic oxygen ions. The results are applied to storm-time ring-current region parameters. The loss of energetic particles resulting from wave-particle interaction may provide an alternate mechanism, complimentary to and independent of the charge exchange process for the ring-current decay. [ABSTRACT FROM AUTHOR]