Will oak tree leaves kill gr?

Someone I work with told me that if you leave oak leaves on the ground or mulch them and they are absorbed by the soil that it is toxic to the gr and the gr will not grow as good around the tree. Is there any truth to this. I have a few pin oaks in my yard and the gr around them could be better. Didn't know if this is why or if it's the shade or spotty fertilizer?