Oaks are the classic coppice tree, so you can and should hack away at it. Cleaning out the competition should give it a new lease on life. If any branches are noticeably colonized by fungi, you can remove those as the tree will do it on its own anyway.
There is really no point in giving it any fertilizer. A tree that old is going to have an immense mycorrhizal network established underground, and it can get the nutrients it needs without any help from you. Just make sure that there is plenty of leaf litter and biomass on the ground to be resupplying those nutrients.
Location: Port Elizabeth, South Africa (34 degrees south)
posted 5 years ago
Thanks John. This helps. What about the wood borers? It looks to me like they are eating into growing wood.