We present measurements of the stable isotopes of oxygen (18O and 16O) from seawater samples collected during the Antarctic Deep Outflow Experiment (ADOX) cruises in the Southern Ocean and southern Indian Ocean, February to March, 1993 and 1994. The data are used in conjunction with hydrographic data to infer characteristics of the formation and circulation of water masses found in the region. The waters on the continental shelf of Antarctica (adjacent to the Princess Elizabeth Trough; PET) are isotopically the lightest found due to the injection of about 1% of glacial meltwater, and are probably advected to the region from farther east by the current associated with the Antarctic Slope Front. They appear to be locally disassociated from the Antarctic Surface Water and Winter Water (WW) farther north in the PET. The WW of the Enderby Basin is isotopically lighter than the PET WW and also fresher, indicating the presence of an additional component of glacial meltwater or high-latitude precipitation. North of the Antarctic Circumpolar Current, the surface δ18O values correlate strongly with salinity, but extrapolate to an apparent freshwater endmember which is isotopically too light to be reasonable; advection and mixing of the water masses dominate over the local water balance at this location. The Subantarctic Mode Water of the southern Indian Ocean lies on the line of the surface waters in salinity-δ18O space due to its local formation by deep convection. The Antarctic Intermediate Water also lies on this line, but it is known that the major sources for this water mass are outside the region. Consequently this observation does not necessarily imply formation by local deep convection. The δ18O measurements have shown that the major sources for PET Antarctic Bottom Water (AABW) lie outside the Weddell and Ross seas. Based on the potential temperature-salinity of the PET AABW we further deduce that the PET itself is not the formation location of a significant amount of AABW. Its major component is advected to the region from the Australian-Antarctic Basin farther east.