My entire property is covered with oak trees, oak leaves, and acorns. From what I have ascertained from vets and reading on line. Oak poisining in the US in horses is rare. Tannins in the oak is what is assumed to be toxic. If there is nothing else to eat they may ingest a toxic quantity. From what I have looked up, while all oaks are potentially toxic, most livestock poisoning is releated to Gambels Oak, Shinnery Oak and Shallow Lobed Oak. Oaks are most toxic in spring and ripe acorns are less toxic than green acorns.
I am not an expert just picked this up from my own concern. Knock on wood, I have not had any toxicity in my horses from the numerous oak leaves and acorns in the pastures.
There's some info on oaks on this site from the Ohio State Univ ext svc. http://ohioline.osu.edu/b762/b762_25.html