There is only one file type that uses the .OXT extension:
Apache OpenOffice Extension
Extension that adds additional functionality to Apache OpenOffice (formerly OpenOffice.org) and StarOffice programs; provides extra features such as advanced spell checking, report building, file compression, and many other capabilities.
To install an OXT file, select "Extension Manager" from the Tools menu, then click "Add" and select the extension from the file browser. If you have Apache OpenOffice 2.2 or later, you can also install extensions by simply double-clicking the OXT file.
OXT files may be developed for the Apache OpenOffice suite or for individual programs, including Writer, Base, Calc, Chart, Draw, Impress, and Math. They may be included with the program or developed by users.
You can download extensions at the Apache OpenOffice Repository.
Updated: June 22, 2015