We have measured the cross section for the dissociative excitation of the forbidden OI(2p4¹D—3s³S, λ1641.3 Å) transition by electron impact on O2. At 100 eV, we find a cross section value of 1.22 × 10−23cm² ±35%. This result is consistent with recent estimates of this branching ratio from theoretical calculations and aeronomical observations and supports the suggestion by Meier et al. [1985] that observation of the OI λ1641.3 Å line would be useful for remote monitoring of atomic oxygen in the uppe