This paper presents searching approaches and user interface capabilities of DUO, an Online Public Access Catalogue (OPAC) designed to permit the users of three Universities of the Northeast of Italy different subject searching accesses to the co-operative multi-disciplines library catalogue database.The co-operative catalogue database is managed by one of the software systems developed under the italian national project for library automation: the SBN project. Since the SBN database has not been designed to be efficiently accessed for end-user searches, the DUO database has been designed to avoid duplication of the SBN database data and to be usable for making efficient subjects accesses to the catalogue documents. The DUO design choices are presented, in particular the main choice of designing a “virtual” document that corresponds to each SBN document and that has unstructured data usable for subject search purposes.The paper presents a new kind of user-OPAC dialogue that makes available to the user different search approaches and on-line dictionaries. In particular the user during the interaction with the search tool can represent his information needs with the support of interface capabilities that are based on retrieval path history, and words and codes on-line dictionaries.DUO is the first Italian OPAC that has been made openly available to users of universities and research institutions. For this reason, it is also the first time that OPAC log data is going to be collected in Italy. This work mainly intends to make a modern OPAC available to the users of a SBN catalogue database, but it is going to permit also to build up a knowledge on OPAC usage in Italy.