How many travelers at Chicago O'Hare Airport are resident's of the
Chicagoland area?  I.E. What % are "passing" through as opposed to
those leaving Chicago (their home) or simply returning home?

http://angryflower.com/aposter.html

This, from Chicago's Commissioner for Aviation,  is not quite what you
are looking for, but it may be helpful to you anyway:

"ROBERSON: No, because it's all a connected system. And that's why
what we have to do is we have to expand and moderniz[e] O'Hare, to put
the infrastructure in place. That way as planes are flying -- because
you have to keep in mind, 50 percent of the 77 million passengers that
we see every single year are connected. So they're coming from
somewhere else through Chicago in order to get to a final
destination."

CNN Student News: Transcripts
http://cnnstudentnews.cnn.com/TRANSCRIPTS/0410/21/ltm.04.html