This baseline dataset comprises downcore measurements of ex-situ oxygen for a sediment core (ROV-operated PUC) retrieved in the framework of the European collaborative JPI Oceans MiningImpact project onboard R/V SONNE cruise SO268 in 2019. The scientific work during cruise SO268 was part of the second phase of the MiningImpact project (Environmental impacts and risks of deep-sea mining) and is designed to assess the environmental impacts of deep-sea mining of polymetallic nodules in the Clarion-Clipperton Zone (CCZ) in the Pacific Ocean. Ex-situ oxygen was determined onboard using amperometric Clark type oxygen microelectrodes (Unisense, DK).