No! Acorns are quite poisonous to horses and can cause colic, edema and kidney damage. Fortunately, the occasional ingestion of a single acorn, such as one that fell off an oak tree, doesn't cause problems, but some horses will develop a taste for them and poison themselves. This is why you sometimes see oak trees fenced off in pastures (cows are even more sensitive to acorn poisoning). Fallen oak branches should always be removed from pasture as soon as possible.
Treatment is activated charcoal and IV fluid therapy.