We have measured the cross section for the dissociative excitation of the forbidden OI(2p4 1D--3s3S, &lgr;1641.3 ¿) transition by electron impact on O2. At 100 eV, we find a cross section value of 1.22¿10-23 cm2 ¿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 &lgr;1641.3 ¿ line would be useful for remote monitoring of atomic oxygen in the upper atmosphere.