OutlineMarkupLanguage (OML) is supposedly to be a vendor neutral, ResourceDescriptionFramework based substitute for OutlineProcessorMarkupLanguage (OPML).
This format is said to be built on OPML syntax, with the addition of the "item" element (child of "outline") so customized attributes are no longer needed. the result is a document that can be better processed by tools. See http://minghong.dyndns.org:8080/OpenWiki/ow.asp?OML
Resources
OPML vs OML http://oml.sourceforge.net/cgi-bin/blosxom.cgi/2003/04/16#debate2
Caution: OML is also the shortform for Ontology Markup Language, which is an XML version of Simple HTML Ontology Extensions(SHOE).