Presentation about the ONTOLISST project at the Challenges of Interoperability session at the 17th Annual European DDI User Conference The diversity of metadata structures in European social science archives – ranging from differences in granularity and provenance to divergent terminology – is a major obstacle to thematic interoperability. This heterogeneity curtails the FAIRness of social science data and hampers their effective reuse across repositories, languages and disciplinary borders. The ONTOLISST project (funded by the OSCARS call of the European Union) tackles this problem not by merely cataloguing the difficulties, but by research-driven, AI-supported exploration of concrete solutions. The session will present the project’s twofold strategy, showcase early results, and open a round table on the particular difficulties that arise when trying to achieve interoperability in social‑science digital repositories.