Kelly I am not an expert on trees, but from my knowledge about lichens on oaks - I think that they are not harming your oaks in any way, but are living there because it offers the right kind of environment for their growth.
Lichens are a algae and fungal combination that grow on surfaces that have some moisture.
The algae needs that moisture to be able to collect photosynthetic wavelengths of light and turn it into energy - the fungi supply a platform for the algae to live on and benefit from the energy produced by the photosynthesis of the algae. They bring different things to the table which enable both life forms to survive, but they are not really stealing anything from the tree or rock or soil that they live - it is just that those tree branches happen to have the right set of environmental conditions favorable for growth of a lichen. That is what I understand about lichens. Maybe someone who knows more about live oaks can offer more insight, but I don't think you have anything to worry about. Thanks for the question.