Oxford Reference Online Oxford Reference Online (ORO) is an online collection of authoritative language and subject reference works (such as dictionaries, encyclopedias, and thesauruses) from one of the world's biggest reference publishers: Oxford University Press. Short entries include brief, factual information (e.g. illustrations, maps, timelines, etc.) while longer entries provide in-depth subject coverage and often link to related ORO entries. The Core Collection (ORO) is a fully-indexed, cross-searchable database; the Premium Collection (OROP) provides added functionality and more detailed information across a broad subject range from titles in the world-renowned Oxford Companions series. For a complete list of subjects and books included in the Oxford Reference Online collection, consult the Books and Subject listings.