While you have ample time between landing at O'Hare Airport to get to Milwaukee for the Indian Trails departure, you can save yourself traveling into downtown Chicago to the main Greyhond or train station. Using one of these options:
- Coach USA buses go directly from O'Hare Airport to Milwaukee Intermodal Station about once an hour. $30 per person one way.
- Greyhound is a bit cheaper: they have a few buses a day from Chicago Cumberland CTA to Milwaukee Intermodal Station. Cumberland is three stops on the Blue Line L/metro outside of O'Hare.
From there you can catch the overnight Indian Trails bus north. You'll have a lot of time to kill before leave the airport for Milwaukee or in in Milwaukee.