Red oak borer?
This is a an internal piece of dead red oak that was that was in the debris when the dead tree was removed. I am thinking that it was done by red oak borers. Can you confirm? Also are those stringy things frass tunnels? Can you explain a little about how they are made? Thank you.
Once a tree is dead, if the death is caused by an insect, the insect leaves the tree because it lives on living plant material. An entirely different cast of insect characters moves in who live on dead wood. Many bore in the wood.They, along with fungi and bacteria, eventually will completely decompose the wood until it is compost or humus.
So, it is impossible to say if the oak was killed by borers or these boring holes were made after it died. In fact, it is impossible to say what killed the tree when it has been dead this long.
If the "stringy things" you see are fragile and crumble instantly when you touch them, they may be frass. There are some ambrosia beetles which produce frass that look similar to this, but the frass tubes are so delicate that rain crumbles and washes them away.
On the other hand, if these "strings" are sturdy, tough strings, they may be the roots of fungi or other plants growing through the dead wood.