The third Orbiting Astronomical Observatory will be launched in the Fall of 1970 carrying a 92 cm telescope and a scanning spectrometer. The instrument can produce stellar spectra in the range from 1000 to 4000 Å at resolutions of 2, 8 or 64 Å. Several entrance apertures can be used, including a 5′ × 6′ slot, a 1′ circle and two narrow slits for special conditions. The spectrometer employs six pulse counting detectors spaced at 500 Å intervals, and can also be used as a fixed wavelength six channel photometer for faint objects. The detection and data handling systems are designed to yield photometric data at a constant statistical precision of 1.6% with a maximum integration time of 42 s at each wavelength.