Yes, oak trees are toxic for rabbits, although not as much as the leaves. But sometimes rabbits are seen munching on these leaves. Rabbits enjoy the dried-out leaves. But if they are eating less, there is nothing to worry about that. Like those rabbits who live in forests or wild rabbits, they don’t get any guidance on what to eat and what not to eat, so they take care of themselves. They also try new things to eat, and by that only do they get the awareness of which leaves are good and which are not. Moreover, it can sometimes help to get them eating again if they go off feed. Just make sure that the leaves are green and there are no insects or fungus that can affect the rabbits' health, and also check that they haven’t collected any insects.