The Oi2b isotope event is a significant Late Oligocene global cooling period for which physical evidence and knowledge are scarce. It is represented by the glaciomarine Polonez Cove Formation (PCF) on King George Island (South Shetland Islands), the oldest and most intensively studied Cenozoic glacial deposit exposed on Antarctica. The PCF is overlain by the much less well-studied, slightly younger Boy Point Formation (BPF), which also preserves vital environmental information. The lithofacies are dominated by phases of coeval volcanism. Autogenic and allogenic processes are identified for the first time and used to determine the history of both formations. The PCF documents the ice terminus, its retreat and sub-polar thermal regime. The recession was followed by isostatic uplift, then by continuous sea level rise due to eustasy. The South Shetland Islands supported an ice dome probably detached from another on the Antarctic Peninsula and separated by at least seasonally open water (proto-Bransfield Strait). The existence of open water in the Weddell Sea explains the presence of erratics dumped by icebergs sourced from East Antarctica and the Antarctic Peninsula, and the study has enabled the configuration of Oi2b ice across Antarctica to be reconstructed for the first time. Rapid uplift terminated marine deposition, helping terrestrial ice to accumulate during a hitherto-unrecognized advance on King George Island. Wet-based ice of a further, slightly younger glacial was probably responsible for deep erosion that truncated the BPF and PCF, cutting >200 m down to present sea level.