Acute Oak Decline and Chronic Oak Decline are terms used for the large numbers of older trees (above 50 years) which are showing signs of dieback and stress. AOD is affecting increasing numbers of Oaks each year and can kill the tree within 5 years. This blog does not all copying. Go to Google either of those diseases there is plenty of info there.
The affected trees are mainly in the eastern counties, midlands and south of England. There has been much research on the causes but recent work has suggested AOD and COD are the same disease at different stages of the spectrum. There was a theory that the Agrilus Beetle was part of the problem but it is more likely the beetle activity is taking advantage of already weakened Oak stems.
More recent research is looking at the soil environment and there has been found a strong link between high Nitrogen levels (from farming/air pollution/car exhausts) in the soil and the mapped trees that are showing symptoms of the disease.
What may be happening is the altered soils are affecting the essential link between mycorrhiza fungi and the roots of Oak trees. The roots need the fungal association to help with water uptake. When this link is severed, then the trees die from lack of water, particularly in the already drier parts of the country such as East and South. 
There is a worry that essential fungi are dying and soils generally are being degraded.