I know from our own sheep that quite small quantities of green oak leaves will cause gut ache and then a black, watery scouring, and I have no doubt that this would be fatal if the sheep concerned were allowed to continue eating such leaves. The poisonous agents are essentially the tanins, which turn to tannic acid in the digestive system and cause ulceration and bleeding, and my veterinary wife tells me other longer term things as well. This link gives a bit more:
and another one from NADIS with reference to cattle, but they assure me that it applies to sheep as well:
I'm not certain, but on our holding it appears that only green leaves and green acorns are attractive to sheep. Our oak trees all have the traditional "browse line" at about 1.7 metres as a result of Texel ewes standing upright on their back legs, and jumping to get the lower leaves; it seems that the quantity they get that way is not enough to cause obvious problems, but a single leafy branch within easy reach on the ground is a real risk. I often resort to cutting a bit of browse for the sheep in August (when I need to trim trees & hedges anyway) but Oak is a real NO, and branches get kept away from sheep until the leaves have gone brown and unpalatable (and some sources claim that the tanins will have been partly leached out, but I have my doubts about that.) A couple of weeks ago I saw a mallard on the moat partly diving in order to porpoise up and grab green acorns of the low branches of an oak; I wonder if they are significantly poisonous to ducks? Pigs can clearly eat quite a lot without problems.