The German National Research Data Infrastructure (NFDI) aims to harmonize the flow of data from science andresearch. Consequently, the NFDI4Memory consortium, responsible for data from historically oriented sciences,also pursues this bigger objective. The goal is to realize a FAIR digital research environment in which users candiscover content through shared semantic structures on previously unconnected materials.Essential to this effort is the development of the NFDI4Memory Ontology (MemO) and the NFDI4MemoryKnowledge Graph (MemO KG). In combination, they build the ground for the NFDI4Memory Data Space,supporting federated searches and semantic interoperability. As a modular extension of the interconsortialmid-level ontologies NFDIcore and the Culture Ontology, MemO incorporates domain-specific concepts fromthe historical sciences, among others, including the harmonization of metadata and the detailed representationof provenance. The MemO KG serves as a central index, harmonizing metadata for research data, institutions,researchers, and services. This infrastructure lays the groundwork for a unified point of access to research dataacross disciplines and consortia. Thereby, it fosters new modes of exploration and research data acquisition inhistorical research.