The only thing I can get python omnicomplete to work with are system modules. I get nothing for help with modules in my site-packages or modules that I\'m currently working
I think your after the pydiction script. It lets you add your own stuff and site-packages to omni complete.
While your at it, add the following to your python.vim file...
set iskeyword+=.
This will let you auto-complete package functions e.g. if you enter...
os.path.
and then [CTRL][N], you'll get a list of the functions for os.path.