The electromagnetic proton cyclotron anisotropy instability is driven by T⊥p/T∥p>1 where p represents protons and the directional subscripts denote directions relative to the background magnetic field. Fluctuating field growth leads to wave-particle scattering, which in turn imposes an upper bound on the anisotropy of the form T⊥p/T∥p-1=Sp/&bgr;∥p&agr;p, where &bgr;∥p≡8&pgr;npkBT∥p/Bo2, and the fitting parameters Sp≲1 and &agr;p≃0.4. Recent SOHO observations indicate that minority heavy ions are substantially hotter and more anisotropic than protons in the solar corona. Here linear theory and hybrid simulations have been carried out in a model of a homogeneous, magnetized, collisionless plasma with anisotropic minority oxygen ions (denoted by subscript O). These calculations show that the electromagnetic oxygen ion cyclotron anisotropy instability also leads to wave-particle scattering, which constrains that anisotropy by the form T⊥O/T∥O-1=SO/[(mp/mO)&bgr;˜∥O>&agr;o, where &bgr;˜∥O≡8&pgr;nekBT∥O/Bo2, So~10 and &agr;O~0.4. This constraint should be observable in the solar corona. ¿ 2001 American Geophysical Union