OpenOffice Writer by default saves the document with file extension as .odt. If you see the document getting saved with out any file type then you can check option available part of the Save/Save As dialog box.
Click File menu then select Save or Save As option from the menu list.
Make sure to mark the check box with label as “Automatic file name extension“. If it is already checked then try marking and un marking the check box again. This should show the default file extension after the file name.