Coming from ORD, you'll hopefully be on a mainline jet. Out of Boston, the RJ's always deposit you at a remote terminal where you walk through a chilly outdoors corridor and then a bus to the main terminal.

After you clear customs and immigration, go upstairs. Most likely, you'd want to re-check in to get a CX BP and a lounge pass. The BA lounge is nice, but a long walk from the actual gates so give yourself about 10 minutes to get to the gate.