I haven't had horses on a property with oaks however I did find this article: Ask the Expert -- Poisonous Plants
Seems that cattle are more prone to oak poisoning and horses will generally leave them alone when given the choice, that is, there is plenty of good feed around for them to eat instead. It also says that Autumn and Spring are the danger times.
Mostly I just wanted to say that I am glad you found a place and to welcome you back to Texas - yee haw!!