Ion conics are commonly observed along auroral zone field lines and involve all major terrestrial ion species, including hydrogen, helium, and oxygen. It is believed that low-frequency plasma waves, driven unstable by field-aligned currents, transversely heat the ion distributions via wave-particle interactions, creating the ion conics. Considered here are low-frequency, oblique, electrostatic instabilities found in a mixed plasma that includes an electron beam streaming through a background of electrons, hydrogen, and oxygen. The addition of oxygen not only modifies the lower hybrid frequency, but allows the existence of an ion-ion (Buchsbaum) hybrid mode with a frequency between the hydrogen and oxygen gyrofrequencies. Because of its low frequency, the ion-ion hybrid instability can be effective in transversely heating heavy ions. When the electron beam drift speed is greater than 3 times the background electron thermal velocity (V0>3vte) and the electron gyrofrequency to plasma frequency ratio is less than 10 (&OHgr;e/&ohgr;pe20, which is a condition commonly found in the auroral zone nightside region, the ion-ion instability has the largest growth rates; in these regions, heavy ion transverse heating can occur. When 10<&OHgr;e/&ohgr;pe<20 the hydrogen to oxygen density ratio determines which instability dominates. ¿ American Geophysical Union 1988