I know this is an Oak, but what species? It does not look to be a Red, nor a Pin. I am thinking White. Second phots is a big old Ash that is so hollow it is beginning to fracture upward and is splitting the bark. The Ash is North of 100 ft tall and 28 dbh. There is barely 4 inches of the outer ring left at ground level. If I tried, I think I could get inside the thing through the crack shown in the pic. There is another opening 90 degrees from this one, but only big enough to reach into. I may try and drop this one tomorrow. It is a tad too close to the house and will likely take out a fence and planted Maple tree if it goes in its' lean direction.