Experts and BHG readers answer.
Black/brown spots on oak tree leaves
I'm sorry, but it's impossible to diagnose this problem without seeing it. The problem you're describing is not something common on oaks. (If it was, we'd know it immediately.) Your best bet is to take a small branch with leaves showing the spots into your local garden center and ask if they've got a Certified Nursery Professional on staff to diagnose it. CNPs pass tests to gain their titles and work on ongoing education to maintain them. They work in the field and have first-hand experience with the insects and diseases that are attacking trees locally. Alternatively, you might contact your local university extension service to see if they have a plant clinic for diagnosis.