The interaction of the thermosphere and ionosphere is largely governed by collisions between ions and neutral particles. On Venus and the Earth, O+ is a dominant ion, and atomic O dominates throughout much of the thermosphere; therefore an accurate O+-O cross section is an important prerequisite for understanding the dynamics of planetary upper atmospheres. The cross section and momentum transfer collision frequency are calculated with a quantum mechanical code which includes resonance charge exchange, polarization, and charge-quadrupole effects. Our results agree well with earlier calculations of Stubbe [1986> and Stallcop et al. [1991>. ¿ American Geophysical Union 1993