You state in the original post that all symptoms, including the O2 lean code, occur when turning on the cruise control. Check the cruise control servo diaphragm and it's vacuum lines for leaks and proper connection.
Er, it's my understanding that fresh air will enter the exhaust at a leak. In fact, I've witnessed this on a number of occasions, mainly through exhaust manifold leaks at the mating surface to the head. The bank with the exhaust leak always has a rich biased fuel trim problem.The exhaust is under pressure. If there is a leak air cannot enter, much like a leak in a garden hose. Unmetered air can only enter via an intake (vacuum) leak.