How to "cat" or "less" a libreoffice writer .odt file?

The solution is using odt2txt. Now, this command is provided by two different packages, namely one called odt2txt that you can install with

sudo apt-get install odt2txt 

and by the package unoconv (that offers also command-line conversions between more libreoffice formats), that is installed by

sudo apt-get install unoconv

If you have the two, you can switch between them using the alternative mechanism:

sudo update-alternatives --config odt2txt

If you are using the odt2txt provided by the package odt2txt you just use

odt2txt file.odt

if using the unoconv provided package you have to use

odt2txt --stdout file.odt

Pipe them to less to have a less-like experience (odt2txt file.odt | less)

Notice that if you do not use the --stdout option, the unoconv-provided package will write the result in a file called file.txt.

The suggested package by the distribution seems to be the one from the odt2txt package (it has a higher default priority in the alternative system); for sure it has less dependencies.

You can browse through text without any special utility. ODT file is a renamed zip archive. Unzip it and you will see several files. One of them, content.xml contains all text and is more or less readable.