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.
Not as long as the wood isn't infested with Brettanomyces, anyway.