Oak trees tend to get hollow spots in them after a period of years. This one is actually trying to heal it's self per the new growth in all three pictures. Looks like the tree was damaged at sometime by lighting or something else. Do not put any materials to seal the spots because that causes water retention and faster decay. If the tree is valuable to you, I would recommend leaving it alone and take good care of it. Keep it on a good watering schedule and use the product made by Bayer Advanced called Tree and Shrub Protect and Feed to fertilize the tree and an insecticide into the tree for protection.