O'Hare is a strong layover airport when you use the CTA Blue Line and keep the plan simple. Downtown is possible, but the round trip can consume a surprising amount of usable time.
What to do with bags
Treat carry-ons as your responsibility unless your airline accepts them for the onward flight. If your bags are checked through, keep your essentials with you and avoid reclaiming anything unnecessarily.
Leaving the airport
The Blue Line gives a predictable path toward Logan Square, Wicker Park, the Loop, or nearby food stops. Pick one area on the same train line and resist cross-town plans.
Getting back through security
Terminal changes can add time, especially if Terminal 5 is involved. Keep your onward boarding pass and ID ready, and return before the checkpoint line becomes the plan.