Ants in the Oak Tree

I did not know that ants can, and do, live in trees. I decided to move the compost heap today so we can put a picnic table (instead of compost) under the nice oak tree in the meadow. I also wanted to get all the parasitic mistletoe out of the nice oak. While up on the stepladder, I put my hand on a little stubby dead bit for balance and approximately 10,000 red-and-black ants came streaming out of it! I had already noticed some strange growths or attachments on some of the branches, about the size and shape of plums and seemingly made of wood; when I cut one open, it, too, was full of ants. Very weird. Lastly, at the base of the tree, there is a stump of what was once a third main trunk, with a hole in it. I was curious. I got a stick. I put the stick in the hole. Out came another 10,000 ants!
I am alarmed and intrigued. Are the ants there because the tree has those dead bits, or are the dead bits dead because the ants are there? Inquiring minds want to know....