For beer aged either in oak barrels or on oak chips/cubes, does the contact with the oak noticeably affect final gravity, and if so, what is the direct cause?
Not really. In a discussion of the subject on the AHA forum recently, it was brought up that using barrels will increase the "angel's share"...the beer that evaporates. But it's such a small amount that it's negligible in terms of increasing the alcohol. Using chips or cubes should have no effect whatsoever.