The file association is set to open odt with Abiword because that is the
default setting for Xubuntu. Abiword comes pre-installed as part of
Xubuntu. Any default file associations can be changed. You can have both
installed and have one open one type of file and another type.
The easiest way to change file associations is from the file manager
(Thunar). Right click on the file type that you want to change, say my
file.odt. it will say Open with 'abiword' or whatever the default
application is. Go down one line to Open With and it will have a side arrow
with a drop down to display all possible applications. Go to the very
bottom and choose Open With Other Application (do this even if it is in the
drop down list). You will get a pop up window showing the menu. Choose the
application and tick the box saying Use as default for this kind of file.
Then choose Open. The reason you do not choose it from the top of the list
of applications should now be obvious. You need to have the option the
change the default and need the box to tick.

The other way to change file associations would be in the Settings. I am
not in XFCE at present so can't step you through that. The other way is
easier and works in most desktop environments.