I do not use LightDM, but I think I have found solution:
1. Your $HOME/.xsessionrc file should have following three lines in it:
if [ -r ~/.profile ]; then
. ~/.profile
fi
If you don't have ~/.xsessionrc file, simply create it.
(Just in case - $HOME and ~ both means your home directory, that is - in most
cases - /home/your-username/.)
2. In your ~/.profile file, put following line:
OOO_FORCE_DESKTOP=gnome
If you don't have ~/.profile file, again - create it. If you happen to have some
conditional statements or loops in that file, put that line at the very end of
file.
Now log out, log in and try running LO.
Below is explanation/rationale, although you may find it a bit technical:
LightDM on login reads script set in session-wrapper in
/etc/lightdm/lightdm.conf file. On Debian, this is /etc/X11/Xsession.
That script sets up X environment, by reading files in /etc/X11/Xsession.d
directory. One of them sources users' XSESSIONRC file, which is ~/.xsessionrc.
In my solution, I tell to put there statement, that reads ~/.profile if it
exists and is readable. Then I tell to put OOO_FORCE_DESKTOP in that ~/.profile
file.
So, in fact, you could put OOO_FORCE_DESKTOP in ~/.xsessionrc and it should
work. I suggest to put in ~/.profile file, because that way you can have all
environment variables in that one single file. If you ever happen to switch
from LightDM to other graphical login manager, or decide to not use any
graphical login manager at all, all your variables should be in place (or will
be with minimal configuration).