The CALLISTO ontology models the domain knowledge and geospatial semantics of the EU-funded CALLISTO project. CALLISTO aims to bridge the gap between Copernicus Data and Data and Information Access Service (DIAS) providers and users in different domains by providing AI solutions that effectively add value to large amounts of satellite data. The project focuses on the Earth Observations (EO) domain and its relationship with four Pilot Use Cases (PUCs): 1) Common Agricultural Policy (CAP) monitoring, 2) water quality assessment, 3) air quality assessment and journalism, and 4) land border surveillance. The CALLISTO ontology has been developed to handle this multi-domain data by offering a semantic representation of each domain and the connections between them. The ontology is reusing the CANDELA ontologies to represent geospatial data and satellite imagery. Meanwhile, additional concepts are introduced in the CALLISTO ontology to accurately capture the domain-specific definitions associated with the targeted PUCs. The objective behind developing the ontology is to convert the domain knowledge into a format that machines can understand and process, typically in the form of a Resource Description Framework (RDF). This transformation enables the application of data analytics techniques and the extraction of additional knowledge through automated inference. Applications such as geospatial question answering, geospatial data retrieval, and cross-domain semantic data-driven applications could use it as an underlying data source. The CALLISTO ontology is available in an RDF format; it is associated with Work Package WP6 within the CALLISTO project, which has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 101004152. For further details, please refer to deliverable D6.1: The CALLISTO ontologies and semantic indexing. The ontology and knowledge graph were developed by the Fraunhofer Institute for Intelligent Analysis and Information Systems (IAIS). In collaboration with PUC members, the ontology was developed and evaluated. Following that, the task of incorporating data from PUC teams into the ontology resulted in the creation of the CALLISTO knowledge graph.