While you have ample time between landing at O'Hare Airport in the morning to get to Milwaukee Intermodal Station for the evening Indian Trails departure, you might want to save yourself traveling the wrong direction into downtown Chicago to the main Greyhound station or train station.
- 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 Airport (much closer than the main Chicago Greyhound stop and on the way to Milwaukee).
These buses will take you directly to Milwaukee Intermodal Station in Downtown Milwaukee, where you can catch the overnight Indian Trails bus. You'll have a lot of time to kill before leaving the airport for Milwaukee or in in downtown Milwaukee between buses (Milwaukee Intermodal Station has a nice waiting area, free wifi, and there is a coffee roaster across the street.)