See: http://www.openarchives.org/OAI/openarchivesprotocol.html#MetadataNamespaces
OAI-PMH supports the dissemination of records in multiple metadata formats from a repository. The ListMetadataFormats request returns the list of all metadata formats. metadataPrefix arguments are used in ListRecords, ListIdentifiers, and GetRecord requests the retrieval of records, or the headers of records that include metadata in the format specified by the metadataPrefix. For purposes of interoperability, repositories must disseminate Dublin Core, without any qualification. Therefore, the protocol reserves the metadataPrefix 'oai_dc', and the URL of a metadata schema for unqualified Dublin Core, which is http://www.openarchives.org/OAI/2.0/oai_dc.xsd. The corresponding XML namespace URL is http://www.openarchives.org/OAI/2.0/oai_dc/.

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DIDL document

The DRIVER community supports the implementation of the metadataPrefix 'oai_dc' and the metadataPrefix 'didl'. Every DRIVER repository that uses the XML container must support this 'didl' metadata schema. The specification of the 'didl' XMLcontainer can be found in chapter Use of MPEG-21 DIDL (xml-container) - Compound object wrapping page.

<OAI-PMH ...>
  <...>
   <record>
    <metadata>
     <didl:DIDL>
      <didl:Item>...</didl:Item>