OAI4J is an open-source client library for OAI-PMH and OAI-ORE created by the National Library of Sweden. The library is object-oriented in it's design and written in Java. It can be used to harvest metadata from OAI-PMH compliant repositories. It can also be used to create new OAI-ORE Resource Maps from scratch, to parse existing ones and to serialize them to xml.
The Sourceforge page for this project can be found here .