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Antarctic continental‐scale glaciation is generally assumed to have initiated at the Eocene‐Oligocene Transition, yet its subsequent evolution is poorly constrained. We reconstruct changes in bottom water temperature and global ice volume from 0 to 17 Ma using δ¹⁸O in conjunction with Mg/Ca records of the infaunal benthic foraminifer, O. umbonatus from Ocean Drilling Program (ODP) Site 806 (equatorial Pacific; ~2500 m). Considering uncertainties in core top calibrations and sensitivity to seawater Mg/Ca (Mg/Ca)_sw, we produce a range of Mg/Ca‐temperature‐Mg/Ca_sw calibrations. Our favored exponential temperature calibration is Mg/Ca = 0.66 ± 0.08 × Mg/Ca_sw^0.27±0.06 × e^{(0.114±0.02 × BWT)} and our favored linear temperature calibration is Mg/Ca = (1.21 ± 0.04 + 0.12 ± 0.004 × BWT (bottom water temperature)) × (Mg/Ca_sw^{−0.003±0.02}) (stated errors are 2 s.e.). The equations are …