Very easy. Download and install Exuberant ctags (http://ctags.sourceforge.net/) (part of any linux distribution even works on Windows).
In your project directory do this:
# ctags -R .
That generates file named "tags".
Now start up your vim in the same directory. Open a file, enjoy OmniCompletion and other things you can do with tags.
Please not while this is not ideal solution (it completely ignores the context) it gives you at least something. Big advantage is you can do the same with any other language that ctags do support (more than 40 computer languages). Also you can jump to definition (function, constant etc). Read: http://vim.wikia.com/wiki/Browsing_programs_with_tags