According to descendants of my home’s original owner, the trees went up when the house was built. In other words, these three oak trees sitting pretty on three of the home’s four corners, were planted in 1925.
When we bought the house in March 2007, we had the trees trimmed so that dead limbs wouldn’t be coming down on our slate roof. Now one of the trees is showing signs of disease and has a dead limb, with missing bark. And that “dead limb” is about 1/3rd of the tree’s substance.
If anyone has any idea what this is, please let me know? We’ll be calling an arborist on Tuesday (when I can stay home all day), but that’s a long ways away.

The long tall limb that's missing its bark. There are no leaves at the top of this limb.

poor tree

Close-up of the funky looking mess. These trees were trimmed in early March 2007, but this rot appeared in the last six weeks or so.

Close-up of trouble spot.

The second oak tree has this MUSHROOM growing in it. I don't know if this is truly a mushroom, or an evidence of some disease process.

And this is what's sitting in the crook of my "healthy" tree. š
The thing in the crook of your healthy tree is a lichen. Lichens are fungi + algae. Ask the arborist if it means anything per the tree’s health. Those are small lichens in the “trouble spot.” Lichens generally grow on dead wood, but again, ask the expert.
The fungus won’t hurt the tree. Fungi only eat dead wood. The die back is either due to lack of sun on the branch, or a problem with the roots. When you severly prune a tree, the supporting roots die under the soil. Think of a tree like a mirror, what happens above the soil happens below as well. That is why many trees that are severly pruned go over in storms. They have lost their support system. I am not an Aborist, just have 45 bonsai I have cared for for close to 30 years.