Oxygen XML Editor does not come bundled by default with libraries which would allow it to import XML content from the new Excel XLSX libraries:
https://www.oxygenxml.com/doc/versions/18.1/ug-editor/topics/import-excel-new-format.html
This plugin contributing additional libraries which allow the Oxygen XML Editor to import new Excel XLSX stylesheets to XML.
This plugin's "oxygenxml.xlsx.import" folder can either be copied to the "OXYGEN_INSTALL_DIR\plugins" folder or it can be installed as an add on (Oxygen Help menu->Install new add-on) from the location: https://raw.githubusercontent.com/oxygenxml/oxygenxml.xlsx.import/master/addon.xml