What is this? How to remedy?
This is gouty oak gall on your oak. There is no easy solution. The galls will not kill it. You may want to improve the oaks growing conditions: putting a wide circle of mulch around it to hold moisture, keep mowers from hitting the trunk, and prevent soil compaction from mowers (this is important!). Keep all mulch from touching the bark. Keep it back a few inches and no more than 2-3" in depth. Compost and regular lawn fertilization in the fall should feed the tree.
Here is more detail about gouty oak gall from our website: