Changes in ice volume and resulting changes in sea level were determined for the late Oligocene (26–23 Ma, Astronomical Timescale, ATS) by applying δ18O-to-sea-level calibrations to deep-sea δ18O records from ODP Sites 689, 690, 929, 1090, and 1218. Our results show that maximum global ice volume occurred during two late Oligocene δ18O events, Oi2c (24.4 Ma) and Mi1 (23.0 Ma) (inferred glacioeustatic lowering), with volumes up to ~25% greater than the present-day East Antarctic Ice Sheet (EAIS). Ice volume during glacial minima was on the order of about 50% of the present-day EAIS. This is supported by late Oligocene stratigraphic records from Antarctica that contain evidence of cold climates and repeated episodes of glaciation at sea level and grounding lines of glacial ice on the Antarctic continental shelf in the Ross Sea and Prydz Bay. In contrast, composite deep-sea δ18O records show a significant decrease (≥ 1‰) between 26.7 and 23.5 Ma, which have long been interpreted as bottom-water warming combined with deglaciation of Antarctica. However, a close examination of individual δ18O records indicates a clear divergence after 26.8 Ma between records from Southern Ocean locations (i.e., Ocean Drilling Program Sites 689, 690, 744) and those of other ocean basins. High δ18O values (2.9‰–3.3‰) from these Southern Ocean δ18O records are consistent with an ice sheet on the East Antarctic continent equivalent to present-day values and cold bottom-water temperatures (≤ 2.0 °C). These differences suggest a reduction in deep-water produced near the Antarctic continent (i.e., proto-Antarctic Bottom Water, proto-AABW), which were quickly entrained and mixed with warmer (and presumably more saline) bottom-water originating from lower latitudes. Expansion of a warmer deep-water mass and the weakening of the proto-AABW may explain the large intra-basinal isotopic gradients that developed among late Oligocene benthic δ18O records. These conclusions are also supported by ocean modeling suggesting a reduction of deep-water formed in the Southern Ocean, strengthening of deep-water from the northern hemisphere, and decreasing temperatures in high southern latitudes occurred as the Drake Passage opened to deep-water. Low δ18O values reported from deep-sea locations other than the Southern Ocean are shown to bias estimates of Antarctic ice volume, calling for a re-evaluation of the notion that Antarctic ice volume was significantly reduced during the late Oligocene.