We present Polar Ozone and Aerosol Measurement (POAM) III observations in the upper troposphere and lower stratosphere (UT/LS) region. A statistical comparison is performed between POAM and coincident in situ ozone observations from the Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) program, with coincidences occurring over the 9 to 12 km altitude range. The comparison indicates that POAM ozone in the UT/LS is biased high by about 0.03--0.07 ppmv, in good agreement with the results of POAM/ozonesonde comparisons. We show that the seasonal variation of UT/LS ozone in the high latitude Northern Hemisphere, obtained from POAM data, agrees well with previous measurements and theoretical considerations. In the lowermost stratosphere, the ozone seasonal cycle is dominated by diabatic descent of ozone rich air from the midstratosphere during winter. In the tropopause region, changes in the ozone seasonal cycle reflect the transition from lower stratospheric to upper tropospheric air. At 350 K, POAM ozone and water vapor mixing ratios, and equivalent latitudes at POAM sampling locations indicate isentropic troposphere to stratosphere exchange occurs during summer and fall. Variations in ozone mixing ratios are also shown to be consistent with variations in isentropic mixing at 350 K.