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- ODD: A 'One Document Does-it-all' TEI format for simultaneously recording project documentation and meta-schema definition from which you can generate RELAX NG, W3C XML Schema, and DTDs as well as formatted documentation.
- ODRL: an XML-based standard Rights Expression Language (REL) used in Digital Rights Management systems
- Office Open XML: is a Microsoft file format specification for the storage of electronic documents
- OFX: Open Financial Exchange is a unified specification for the electronic exchange of financial data between financial institutions, businesses and consumers via the Internet.
- OIOXML: an XML-markup language created by the Danish government to ease communication from, to and between Danish governmental instances
- Open Mathematical Documents (OMDoc), based on OpenMath and MathML, but with a greater coverage.
- OML: an XML format for outlines, based on OPML.
- Open eBook: the e-book format defined by Open eBook Publication Structure Specification; superseded by ePub.
- Open Scripture Information Standard (OSIS), an XML-markup schema that defines tags for marking up Bibles, theological commentaries, and other related literature.
- OpenDocument: a document file format used for describing electronic documents
- OpenMath - a markup language for mathematical formulae which can complement MathML.
- OPML: an XML format for outlines
Read more about this topic: List Of XML Markup Languages