This is what I use:
# -*- mode: Org; eval: (auto-fill-mode 0) -*- #
Note that the #
characters are optional, I just prefer them for style reasons.
$ info emacs
48.2.4.1 Specifying File Variables:
Do not use the `mode' keyword for minor modes. To enable or disable
a minor mode in a local variables list, use the `eval' keyword with a
Lisp expression that runs the mode command (*note Minor Modes::).`