Now my info may be out of date as it's been a couple of years since I flew back from ORD on an non US based carrier.

The international airlines used to depart from T5 whereas the US carriers departed from their own terminals. This meant that BA, LX and SK etc provided their own lounges within the extremely sparce and boring space that T5 was/is.

Correct me if I'm wrong, but there is no way that you can check in for LX then go to another terminal to get access to CO, US or UA's lounges.......