Question
I have two holes in a large oak tree on a hill behind my cabin.  the holes are about 4 inches in diameter and a crack about 3 ft long.  I haven't looked inside the holes yet (but will when I get up there again).  I am trying to decide if the tree needs to be removed.  It's around 10 to 12 in diameter trunk and atleast 50 ft tall.

Is there anything you can tell me that I should look for to help me decide whether it's time to take it down?

Answer
As long as the foliage is full and healthy looking the tree is healthy. IT sounds like it does have a decay fungus in the woody cell of the trunk BUT decay fungi only effect the dead wood and will not harm the living cells. Generally the fungus will continue to grow slowly and hollow out the tree trunk. It could weaken the trunk and cause it to break during a high wind storm. But before this happen you should see large limbs break off which will also be hollow. If the foliage starts to get sparse and the leaves smaller the tree would be a tree to watch for removal but from what you described it will last many many years as is.