Some word-processing programs based on the OpenDocument architecture also support Microsoft Word formats. These programs can open Word documents (*.docx, *.doc) and simply save them in Open Document Interchange Format. Except for some Word exclusive formatting and features, everything should be converted just fine.
Convert Microsoft Word Open XML document to Open Document Interchange Format (ODIF).
We found 2 software records in the database related to .docx to .odf file format conversion.
Apple macOS / Mac OS X software - convert docx to odf on OS X
A full-featured set of office applications for Mac OS X
Convert from docx file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned docx to odf. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Multiplatform software - convert docx to odf on any platform
Full-featured word processing and desktop publishing tool from LibreOffice suite