Standard ECMA-376: Office Open XML File Formats - Official text of the OOXML file format as accepted by the Ecma International standards body. - http://www.ecma-international.org/publications/standards/Ecma-376.htm
Ecma Office Open XML File Formats Overview - Official details about the format, and links to technical information. - http://office.microsoft.com/en-us/products/HA102058151033.aspx
PC World: ISO Rejects Microsoft's OOXML as Standard - Article providing details of the outcome of the recent ISO voting on Microsoft's document format specification. - http://www.pcworld.com/article/id,136711/article.html
Wikipedia: Office Open XML - Encylopedia article about the forum, including background, standardization efforts, and controversies. - http://en.wikipedia.org/wiki/Office_Open_XML