The setup of outshine package for emacs requires the user to set the
variable outline-minor-mode-prefix to "\M-#" before the outline-mode
package (that ships with emacs) is loaded.
The best way to do this is via the Customize interface in emacs.
So I did M-x customize and set that variable’s value to "\M-#" and restarted
emacs and that worked.
But then I was surprised to see that value being saved in the custom.el as
'(outline-minor-mode-prefix "\243").