The Microsoft Office Open XML (OOXML) File Format is an application format developed by Microsoft and released to the public in 2011. It is designed to complement existing formats such as DOCX and XLSX to allow developers to create and exchange documents that are compatible with Office 365 and older versions of Office. The first version of OOXML was released for Office 2013.

OOXML allows developers to create and manage content easily. Developers can create OOXML document templates that they can use to create the required number of similar documents with a few clicks. It is also possible to create custom controls and layouts to improve the appearance of the documents.

The development of the OOXML format is a cooperative effort by Microsoft and several partners such as OpenXML and LibreOffice.

The OOXML file format can be used for both Windows and macOS. A variety of Microsoft programs can work with OOXML including Word, PowerPoint, Excel, Visio, Outlook, OneNote, and Access. The program is supported on Linux, Android, and iOS.