Motivation: Orphanet serves as a reference portal for rare diseases populated by literature curation and validated by international experts. The Orphanet information system is supported by a relational database designed around the concept of a disorder. Increasingly, Orphanet is seen as a reference for this domain and as such is required for reuse by external applications. These applications require complex queries, specific views tailored to user groups or investigation areas and integration or cross-referencing with resources such as OMIM and Ensembl. A formal, portable and open-access ontological representation of Orphanet is required by the community. Results: We present the Orphanet Rare Disease Ontology (ORDO), an open-access ontology developed from the Or-phanet information system, enabling complex queries of rare disorder and its epidemiological data (age of onset, prevalence , mode of inheritance) and gene-disorder functional relationships. Bespoke views can be extracted using the ontology axiomatisation eg. phenome-disorder views. Availability: ORDO (OWL and OBO format) is available