The open-source OpenOffice provides a fully functional, free alternative to paid office suites such as Microsoft Office. Microsoft's Word program, however, still looms as a driving force in the office suite industry -- its .doc format was a de facto standard for years. Microsoft introduced .doc's heir, the .docx format, with its Word 2007 application and released patches to make older versions of Word compatible with the new format. OpenOffice never released a compatibility patch for older versions of its software; to read and save .docx files with OpenOffice, you'll need version 3.0 or higher.
Launch OpenOffice Writer normally. Create a new document or open an existing file.
Click on "File" in the toolbar. Select the "Save As" option if you're saving a new file or if you'd like to convert an existing file to .docx format. A "Save" dialogue box opens up.
Name the file as desired in the "File Name" field and choose where you'd like to save it on your hard drive.
Click on the arrow to the right of the "File Extension" bar. A menu drops down with several different file extension options. Click on the .docx extension to save the file in that format.
Push the save button to finishing saving the .docx file in OpenOffice.
Simply click "File" > "Save" or press "Crtl" + "S" to save any changes to an existing .docx file without fiddling in the dialogue box.