If you go to /cgi-bin/koha/oai.pl?verb=Identify, you'll see that "Repository identifier" and "Sample identifier" are blank. The ./koha-tmpl/opac-tmpl/xslt/OAI.xslt makes it seem that they should be at oai:description/oai_id:oai-identifier/oai_id:repositoryIdentifier/text() and oai:description/oai_id:oai-identifier/oai_id:sampleIdentifier/text() but there is no oai:description in the XML response. I imagine that someone must've written that XSLT with purpose once upon a time but that data has been lost to the ages...