How to convert OpenOffice .odt documents to Microsoft Word .docx format

Officially, OpenOffice does not support the latest Word format that creates .docx documents.

But there's a solution.

You can use a free built-in program that came pre-packaged with Windows.

How to:
[1] Right-Click the .odt file that you would like to convert
[2] Select Open with > WordPad. If WordPad does not appear on this list, you can select it from the Choose default program menu.

[3] Next Click the File menu to the left of the Home tab.
[4] From the menu list Select Save as > Office Open XML document.

[5] Now, save your file as an .docx-file.

Your file should now be in the .docx format and readable by any version of Microsoft Office Word.

Source.