Parametric decays of large-amplitude electromagnetic ion cyclotron waves (EICW) due to a minor O+ ion component in the magnetosphere are studied. It is shown that the presence of O+ ions leads to a number of new wave couplings which in turn lead to new instabilities. Some couplings involve sound waves carried mainly by the O+ ions, and a sideband EICW which has a resonance at the O+ ion gyrofrequency. These are decay instabilities which can lead to O+ heating through Landau damping and/or resonance absorption. There is also a modulational instability involving two sideband EICW, one propagating forward and the other propagating backward relative to the external magnetic field. These waves can also transfer energy to the O+ ions through resonance absorption. The other branches of the dispersion relation, namely, the He+ and proton branch, have additional decay instabilities (Gomberoff et al., 1995) due to the presence of a minor O+ ion component. It is also shown that in the fluid description, the decays to sound waves associated with the minority heavy ion species have growth rates comparable to, or even larger than, the decays to the acoustic branch corresponding to the majority proton species. ¿ American Geophysical Union 1995