We present the goals and first results of the NFDI Section Metadata Working Group on Ontology Harmonisation and Mapping. The ongoing analysis of the used terminologies within the NFDI consortia suggests that, agreeing on a single NFDI wide ontological framework for very general and interdisciplinary concepts needed in the semantic annotation of research date is not feasible in the short run due to domain specific requirements. We thus present how the Research Data Alliance (RDA) framework I-ADOPT, which focuses only on the formal description of observation variables in scientific studies, could be utilised to provide an NFDI wide and global common ground for data interoperability.