An Odt file is a text document created by OpenOffice and LibreOffice.
Odt files are similar to Word .docx files.

Check file format:

$ file doc.odt
doc.odt: OpenDocument Text

An Odt file is a Zip archive containing xml files.
Let unzip it, and read content.xml file.

unzip doc.odt    
cat content.xml

Or use odt2txt to extract text:

odt2txt doc.odt