Mas afinal o que é o OOXML?
Office Open XML (commonly abbreviated as OOXML) is an XML-based file format specification for electronic documents such as memos, reports, books, spreadsheets, charts, presentations and word processing documents. The specification was originally developed by Microsoft for its Microsoft Office 2007 product suite and was published by Ecma International as the Ecma 376 standard in December 2006. OOXML is currently the subject of a standardization attempt within the International Organization for Standardization (ISO).
Decalcado da Wikipedia