Here's an excerpt from the announcement:
The OAI Toolkit is used to make data stored in an institution's ILS or other repository available for harvesting via OAI-PMH, including other eXtensible Catalog applications. For an ILS, this is accomplished by exporting ILS metadata, converting it from MARC to MARCXML, and loading it into an OAI-PMH compliant repository. The repository (embedded in the OAI Toolkit) makes the data available for harvesting by other XC components.
The OAI Toolkit can be used as part of the XC system, or on its own to enable OAI-PMH harvestability of an existing repository. It is a server application written in Java and is only needed for ILS's and other repositories that do not already have the ability to be act as OAI-PMH Repositories (OAI Servers).