Alas, what used to be the best brunch buffet in the airport area is no more, now that the Sofitel has become a Hilton. I'm not sure whether they do brunch. I'd imagine some of the Rosemont-area hotels must offer something in that line, and it might be a service to check that out. I think the Hyatt Regency O'Hare has a buffet.
Meanwhile, if you want an already vetted spot and are willing to settle for order-off-the-menu diner breakfast, I can highly recommend
Mrs. V's in Des Plaines.
Mrs. V's Restaurant1640 S. River Road, Des Plaines
(847) 297-7185