One of my bunnies used to LOVE oak leaves. She only got limited amounts of it, a leaf here and there while out in the playpen. I never really noticed it make her sick, until one day my dad gave her a whole heap of leaves and sticks to play in and she pigged out on it and came down sick, all not wanting to move or do anything. :/ After that, I don't think I'd deliberately feed oak-anything to them. >.>