If you're interested in encoding not just metadata about oral histories 
but also the transcriptions themselves, you might consult the Best 
Practices for TEI in Libraries ( http://purl.oclc.org/NET/teiinlibraries 
).  There are discussion of encoding oral histories in the section on 
"Level 4" encoding.  There's also a link here to a digital resource 
called Oral Histories of the American South ( 
http://docsouth.unc.edu/sohp/ ) which produced such TEI encodings.  They 
might have developed practices for use of TEI headers for metadata in a 
way that goes beyond what's given for the header in the Best Practices 
for TEI in Libraries.

--Kevin