Seawater samples for stable oxygen water isotopes (δ18O) were taken during leg 1-5 of the MOSAIC expedition either from the Ocean City (OC CTD) on the main MOSAiC ice floe or from the shipboard CTD (PS CTD). Analysis for δ18O where conducted at the Leibniz Laboratory (LLK, Kiel, Germany) with mass-spectroscopy, applying the CO2- water isotope equilibration technique on at least 2 sub-samples on a Finnigan gas bench II unit coupled to a Finnigan DeltaPlusXL. A subset of stations were analyzed at AWI and published separately as Mellat et al. 2023 (https://doi.org/10.1594/PANGAEA.958464).