The oceans have the largest heat capacity in the climate system, therefore controlling the rate of climate change. Ocean circulation redistributes heat, and variability in that circulation determines seasonal to decadal variability in climate. Syntheses of the ocean state using models constrained by observations are critical for understanding climate and predicting climate variability. However, there has been little organised activity evaluating them. Clear information is needed about the strengths and weaknesses of ocean syntheses, as well as guidelines on how fit for different purposes various ocean syntheses are. Recent improvements on Earth observation systems, like the completion of the Argo global float network and the ESA projects on Essential Climate Variables, provide new datasets that can improve the quality of ocean syntheses, allowing ocean variability to be characterised more accurately. The main goals of this COST Action are to improve the coordination of the European efforts in the evaluation of ocean syntheses, to optimize their use and value, to ease their access, to promote their improvement and to raise confidence in their quality. Recommendations and guidelines will be provided on the evaluation, quality and applications of ocean syntheses to end users. These evaluations require cross-disciplinary meetings with experts in Earth Observation, ocean and atmosphere syntheses, air-sea flux measurements and modelling and physical oceanography. This COST Action will provide the optimal framework for integrating these communities.