First, you have to install brew: http://mxcl.github.com/homebrew/

With brew installed on your system, you can install postgresql, python 2.7 and many of the dependencies you can see in the installation manual on doc.openerp.com.

What you can't install with brew is usually installable with easy_install or pip as a python module.

We can assume that you can, more or less, follow the installation guide for GNU/Linux and replace "apt-get" with "brew".