Green oak leaves are acidic. Dried oak leaves are more neutral than anything else. They can even be slightly alkaline in certain conditions. A link
http://extension.oregonstate.edu/que...week/oak-mulch
After years of adding dried oak leaves, the soil PH levels in our gardens are exactly the same. Adding oak leaves is just adding organic matter and some trace elements. It is building your soil organically.
The way I look at this process is I am helping nature do what it does best.