This opens a file in the already running Emacs instance provided you have (server-start)
in your init.el:
alias emax='/Applications/Emacs.app/Contents/MacOS/bin/emacsclient -n'
Usage example:
emax ~/.emacs.d/init.el
Note: I couldn't get the OS X open
command to use emacslient as the default editor. The bash EDITOR variable doesn't seem to work.