Dissolved oxygen in seawater is vital for marine life, diversity, and global biogeochemical cycles. Understanding how oxygen has changed through the geological record offers key insights to understand potential future scenarios. In the past, both the absolute abundance and distribution of ocean oxygenation was at times, different from today. Such changes are driven by different processes, including oxygen solubility, ocean circulation, and/or biological processes. In this session, we invite contributions investigating past seawater dissolved oxygen reconstructions on regional to global scales across the Phanerozoic. This includes studies of drivers and mechanisms of change, feedback, and impacts on ecosystems and the Earth system. Studies focussing on proxy applications or development, modelling, and proxy-model comparisons are welcome.