Oak trees contain tannins although the content varies with the season and part of the tree. There are high concentrations in acorns and there are documented cases of acorn poisoning in other species. In moderation, tannins are not poisonous. They are present in tea. Rabbits will not be harmed if they eat some oak leaves, although they are not a good choice to pick when foraging. It is sensible to avoid feeding acorns although wild rabbits seem to enjoy them without coming to any harm.