Asked June 10, 2021, 7:02 PM EDT
Lucas County Ohio
It is difficult to say without seeing the trees or knowing the variety. Depending on the variety, it could simply be "oak leaf drop". Some oak varieties keep their leaves throughout winter and drop them in the spring.
Are any of the leaves damaged? Discolored? If any of the leaves are damaged or discolored, I recommend taking some of the leaves to to your local Extension Office (find your office by clicking on the link below -- call to see if they are open) or a local reputable nursery and see if they can identify any issues. You may also want to contact a local reputable arborist to stop by and take a look.
I am listing some links to information on some of the more common oak issues. This may help you to identify any problems.
Oak Leafminer: https://bygl.osu.edu/index.php/node/1796
Oak Wilt: https://bygl.osu.edu/index.php/node/1668
Oak Leaf Blister: https://bygl.osu.edu/index.php/node/805
Oak Lace Bug: https://bygl.osu.edu/index.php/node/369
I hope this helps you!