The formula for the O+, O momentum transfer collision frequency has been uncertain due to a discrepancy between results of theoretical calculations and some of the joint radar/optical studies. The former suggest a multiplicative factor equal to 1.2--1.3 times the formula derived by Dalgarno [1964> and Banks [1966>, while the latter suggest a multiplicative factor F=1.7 [Salah, 1993>. We present results of a new analysis of data from 30 nights of coincident incoherent scatter radar (ISR) and Fabry-Perot interferometer (FPI) experiments conducted at Millstone Hill between 1988 and 1992. The O+, O collision frequency is estimated from FPI measurements of the horizontal neutral wind in the magnetic meridian, ISR measurements of the ion drift velocity parallel to the Earth's magnetic field and other data at the calculated height of peak 630 nm emission, and the mass spectrometer and incoherent scatter 86 model. A complete error analysis is carried out for each derived value of F. This allows us to carry out Monte Carlo simulations which confirm that random errors lead to an increase in the mean value of F and which provide us with an unbiased result, F=1.15¿0.2. However, this result was obtained from an analysis which neglected vertical neutral winds, about which we have little information. The most likely effect of these winds would be an increase in the value of F,so that our best estimate from this study is F=1.4¿0.3, which is consistent with theoretical calculations.¿ 1997 American Geophysical Union