The Microsoft Office OOXML Document Type

Office Open XML (also referred to as OOXML or OpenXML) is a free and open Ecma international standard document format, and a proposed ISO/IEC standard for representing spreadsheets, charts, presentations and word processing documents.

OOXML uses a file package conforming to the Open Packaging Convention. This format uses the ZIP file format and contains the individual files that form the basis of the document. In addition to Office markup, the package can also include embedded files such as images, videos, or other documents.

Oxygen offers support for editing, transforming and validating documents composing the OOXML package directly through the archive support.
Editing OOXML packages in Oxygen
A file is considered to be an OOXML document when it has one of the following namespaces:

The schema used for these documents is located in ${frameworks}/ooxml/schemas/main.nvdl, where ${frameworks} is a subdirectory of the Oxygen install directory. The schema can be easily customized to allow user defined extension schemas for use in the OOXML files. See the Markup Compatibility and Extensibility Ecma PDF document for more details.