Oak trees carry the largest amount of insects of any other UK tree, feeding numerous birds, so I don't think there is anything you can do to such a large tree, and would you want to?

Best to ask your local authority Tree Office for advice.
If it is as large as that, it will probably be protected.

It's honeydew from scale insects on its leaves. The best and most effective thing that you can do in this situation is not to park your car or leave any lawn furniture under it:)

we have the same problem with the beech tree. the bees were having a feast on the honeydew, from the millions of greenfly. as its a large plant there isn't anything you can do really.

All we do is make sure not to park the car underneath it...