We demonstrate that the observed enhancement in the fractional composition &eegr;O+ of ring current O+ ions during magnetic storms can have a strong controlling effect on the excitation of electromagnetic ion cyclotron (EMIC) waves. For modest storms, when &eegr;O+≤30%, strong EMIC excitation can occur in the frequency band above the oxygen gyrofrequency, &OHgr;O+, due to cyclotron resonance with anisotropic ring current H+ ions. The path-integrated gain obtained from ray tracing is sufficient to drive wave amplitudes into the nonlinear regime in a region near the equatorial duskside plasmapause. The excited wave energy is found to be absorbed efficiently at high latitudes via cyclotron resonant interactions with energetic O+ leading to perpendicular heating of the O+ population. Intense waves generated near the equator should therefore not be detectable at low altitudes once the density of O+ has been enhanced during the main phase of a storm. Cyclotron absorption will also enhance the anisotropy of energetic resonant O+ ions. We show that such enhanced anisotropy can excite cyclotron instabilities at frequencies below &OHgr;O+ which are able to propagate to low altitudes and be detectable either on the ground or on low altitude satellites during the storm main phase. For the most intense storms, when the concentration of O+ can attain values &eegr;O+≥60%, cyclotron absorption by resonant O+ can become so severe as to totally suppress wave excitation in the band above &OHgr;O+. The most rapid loss process for the ring current (i.e., that due to wave particle scattering) could therefore be suppressed during the main phase of such storms. This raises the interesting question of whether the main phase Dst depression might be modulated by the relative concentration of energetic O+ through the process of resonant interaction with EMIC waves.¿ 1997 American Geophysical Union