The emission line spectrum of singly ionized oxygen (O II) dominates the day airglow spectrum in the extreme ultraviolet below 834 ¿. The strongest resonance line, at 834 ¿, is optically thick and an analysis of height profiles obtained from rocket observations between 140 and 265 km in specific viewing directions indicates that the principal excitaton source is direct photoionization of neutral atomic oxygen. Strong emission at 538--539 ¿ is most likely due to the quartet rather than the doublet transitions, which both occur at these wavelengths and which are not spectrally resolved in the data. The intensities of the weaker lines are consistent with recent laboratory measurements of transition branching ratios. Several strong O II lines near He I 584 produce severe contamination of low-altitude (<400 km) measurements of geocoronal helium emission made with thin-film broadband photometers.