Technically Office Open XML is not proprietary, since they published and standardized it under ISO/IEC 29500. Practically no software suite besides MS Office is able to render it 100% compatible to each other. But to be fair, neither did OpenDocument; a document in LibreOffice odt looks slightly different in OnlyOffice, WPS, SoftMaker, etc.
Technically Office Open XML is not proprietary, since they published and standardized it under ISO/IEC 29500. Practically no software suite besides MS Office is able to render it 100% compatible to each other. But to be fair, neither did OpenDocument; a document in LibreOffice odt looks slightly different in OnlyOffice, WPS, SoftMaker, etc.