jodconverter is a nice library that uses OO to covert between file formats. Since OO understands ODF (of which ODT is part) it might help.
If you want a more direct approach, ODF (and ODT) files are XML files in a zip archive. So you can use the java.util.zip package to open the file, and then process its constituent files using the JAXP API. The file that has the actual content is called "content.xml".