One of the largest solar proton events in the past thirty years took place on October 28, 2003 and had a significant impact on the Earth's middle atmosphere. The incoming protons produce significant amounts of HOx constituents in the mesosphere and upper stratosphere that lead to ozone depletion. For 11 hours during the solar proton event the OSIRIS instrument on Odin measured high spatial resolution profiles of the oxygen infrared atmospheric band at 1.26 microns which under sunlit conditions can be used as a direct proxy for ozone. Ozone depletion is observed across the southern polar cap for the duration of the observations and extends to latitudes as far north as 45¿S. OSIRIS observed significant ozone depletion between 50 and 80 km with a maximum value of 75% around 65 km. The actual maximum depletion could have been even greater as observations ceased while the depletion was still increasing.