There are several bus/coach companies providing service to/from Chicago O'Hare.
Pace Bus links O'Hare with many Chicago areas including downtown. For more information.
The Bus/Shuttle Center is located on the ground level of O'Hare's main parking garage near Terminals 1, 2, and 3. This is the pick-up point for most regional buses, shuttle services and hotel and car rental courtesy buses.
Some bus services also pick up from Terminal 5, the international terminal.