The H+-O+ two-stream instability in a magnetized plasma is examined for application to ion beam heating in the auroral electron acceleration region. In the fluid limit (zero ion temperature) it is found that the most unstable mode coupling is between the obliquely propagating fast O+ and slow H+ cyclotron modes, even for relative ion beam velocities where parallel propagating modes are unstable. The most unstable mode has properties analogous to those of beam-plasma instability. Once a relative velocity is reached where obliquely propagating modes dominate, the growth rate becomes independent of the perpendicular angle of maximum wave growth. Upon adding a finite ion temperature it is found that the gross features of the H+-O+ fluid interaction are retained but finite gyroradius effects cause the appearance of an angle of maximum growth. The tangent of this angle is proportional to the velocity difference between the ion beams, for large enough relative velocity, and is also a function of the H+ to O+ density ratio. The qualitative behavior of the maximum growth rate and its frequency and wave vector versus relative velocity and density ratio are examined. The implications of this behavior for H+ and O+ beam heating in the auroral electron acceleration region are discussed. ¿ American Geophysical Union 1988