OIOXML is a project by the Danish government to develop a number of reusable data components serializable in various formats, although currently the only method of serialization for OIOXML data is in the XML

XML

Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....

 format. This project was undertaken so as to ease communication from, to and between Danish governmental instances. It was made as part of the Danish government's transition to what they refer to as an eGovernment

EGovernment

E-Government is digital interactions between a government and citizens , government and businesses/Commerce , government and employees , and also between government and governments /agencies...

, in which communication between governmental instances, companies and the public should be paper-free. There has been some confusion as to what OIOXML is as the most prominent OIOXML format, the Danish Efaktura format which is a localization of UBL

Universal Business Language

Universal Business Language is a library of standard electronic XML business documents such as purchase orders and invoices. UBL was developed by an OASIS Technical Committee with participation from a variety of industry data standards organizations. UBL is designed to plug directly into existing...

is also referred to as OIOXML by many governmental documents. It is currently a requirement for all invoices given to a Danish governmental organization to be in the Efaktura format.

Sources

• The interoperability framework
• OIO - Offentlig Information Online (public information online) - english main page of the site
• Description of OIOXML and its reasons
• Reference to the OIOXML markup language
• Validator for OIOXML
• Examples of OIOXML invoices in comparison with regular invoices (danish)