On a multi-segment itinerary on a single ticket, check in with the airline operating the first segment. They should issue boarding passes for all segments, so that when you arrive at ORD, you can go directly to the gate for your next segment to NRT. Of course, the main risk here is that your flight to ORD will be late, causing you to miss your connection.

On your return trip, be aware that on your first landing in the US, you will have to:

1. Do US immigration.
2. Collect checked baggage.
3. Do US customs.
4. Re-check checked baggage if you have additional flight segments.
5. Go through security screening to get to the gate for your next flight segment, if any. (Some poorly designed airports have no way to exit the airport if it is your final destination without going through the screened area.)