The forbidden 2p4 1D--3s 3S0 transition of atomic oxygen at 1641 ¿ has been identified in the satellite dayglow spectrum of Huffman et al. (1980). By modelling the optically thick 3P--3S 1304 ¿ transition, a probability of 5.4¿10-6 is determined for 3S0 atoms branching to the 1D state rather than to the 3P ground state. The ratio of the 1641 ¿ to 1304 ¿ intensities is found to be a linear function of the atomic oxygen abundance; therefore the ratio can be used to determine the O concentration in optically thick planetary thermospheres.