We present TAO (The Arcarium Ontology), an ontology that will serve as the backbone of the forthcoming Arcarium knowledge base for storing and retrieving rich metadata of manuscript collections housed at Jagiellonian University (Kraków, Poland). The main motivation for its creation is the ongoing digitization of the precious Autograph Collection from the so-called Berlin Collection, however, the resulting solution is intended to be as universal as possible to allow metadata acquisition in a wide variety of projects. Therefore, the proposed ontology, in addition to the document properties collected by experts analyzing the digitized collection, also includes concepts and relationships from the Europeana Data Model (EDM), CIDOC Conceptual Reference Model (CIDOC CRM), Records in Contexts Ontology (RiC-O), Encoded Archival Description (EAD) and Gemeinsame Normdate (GND), thus enabling integration with external knowledge bases and facilitating advanced research scenarios in the digital humanities. Although still a work in progress, yet the ontology has already been fed with the first real instances---the documents described in the digitization pilot---and discussed with an international panel of experts, which allowed its critical evaluation and indication of its usefulness for the Arcarium system under development.