A large database of sunlit Io [O I> 6300 ¿ emission, acquired over the period 1990--1999, with extensive coverage of Io orbital phase angle ϕ and System III longitude &lgr;III, exhibits significant long-term and short-term variations in [O I> 6300 ¿ emission intensities. The long-term average intensity shows a clear dependence on &lgr;III, which establishes conclusively that the emission is produced by the interaction between Io's atmosphere and the plasma torus. Two prominent average intensity maxima, 70¿ to 90¿ wide, are centered at &lgr;III≈130¿ and &lgr;III≈295¿. A comparison of data from October 1998 with a three-dimensional plasma torus model, based upon electron impact excitation of atomic oxygen, suggests a basis for study of the torus interaction with Io's atmosphere. The observed short-term, erratic [O I> 6300 ¿ intensity variations fluctuate ~20--50% on a timescale of tens of minutes with less frequent fluctuations of a factor of ~2. The most likely candidate to produce these fluctuations is a time-variable energy flux of field-aligned nonthermal electrons identified recently in Galileo plasma science data. If true, the short-term [O I> intensity fluctuations may be related to variable field-aligned currents driven by inward and outward torus plasma transport and/or transient high-latitude, field-aligned potential drops. A correlation between the intensity and emission line width indicates molecular dissociation may contribute significantly to the [O I> 6300 ¿ emission. The nonthermal electron energy flux may produce O(1D) by electron impact dissociation of SO2 and SO, with the excess energy going into excitation of O and its kinetic energy. The [O I> 6300 ¿ emission database establishes Io as a valuable probe of the torus, responding to local conditions at Io's position. ¿ 2001 American Geophysical Union