In this Letter, we report for the first time, the ratio of the O I (135.6 nm)/O I (130.4 nm) absolute emission cross sections from electron-impact dissociative excitation of O2 at 100 eV using facilities located at the University of Colorado, Laboratory for Atmospheric and Space Physics (LASP). The O I (135.6 nm) emission glow produced in the laboratory from the long-lived transition O I g3P←5S0 was measured in a large collision chamber (1 m diameter) with a long focal length (50 cm) electrostatic electron gun. The ratio of the absolute emission cross sections of the O I (135.6 nm) and O I (130.4 nm) features resulting from electron-impact dissociative excitation of O2 at 100 eV was found to be 2.3¿0.6. From this ratio, the absolute emission cross section for the O I (135.6 nm) feature at 100 eV electron-impact energy was determined to be (6.4¿2.2)¿10-18 cm2. The O I (135.6 nm) cross section reported here can be used to better understand the recent Hubble Space Telescope observations of the O I (130.4 nm) and O I (135.6 nm) emission intensities from Ganymede [Feldman et al., 2000> and Europa [Hall et al., 1995>. ¿ 2001 American Geophysical Union