wow, that was one random request. xD
oak trees are like evergreens and their leafs give the surrounding area a lower pH which isnt always good.
different plants prefer different pH to grow in.
maybe you've heard that flowers dont like to grow under evergreens and its the same deal with oaks.
therefore when you compost oakleafs you can put those separately and you will have great nurishment for plants that prefer lower pH like rhododendron and such.
if you mix all tree leafs, depending on the amount of oak leafs of course, you will get a mix of lower and higher pH which is still good but maybe not for all plants.
...if that makes sense? ^^"