The safest time to prune oaks is during winter, so avoid pruning or wounding the tree between mid-March and late June. During this time of year, insects such as leaf hoppers and spittlebugs can carry Bacterial Leaf Scorch (BLS) from tree to tree. They are attracted to the sap which flows freely from wounds.
However you can (and should) remove all dead, diseased and damaged limbs at any time. Just disinfect pruning tools with a 10% bleach solution between pruning cuts to protect against the spread of BLS
Infected trees will have browning which occurs on older leaves and then spreads to newer growth. In spring the tree may appear to leaf out normally, but by midsummer the symptoms will spread further in the tree. Although scorching seems minimal at first, the disease becomes worse every year and can kill a mature tree in 4-6 years. There is no cure for this disease.
Although less common BLS can also affect elm, sycamore, mulberry, sweetgum, sugar maple, and red maple.