Oak, Ash, sycamore are all hebsafe.
Acorns, whilst not poisonous per se, can block the sheep's stomach with all those rounded things, which can't move on, so the sheep starves to death, or gets acidosis (or whatever 'stagnation in't stomach' might be
).
Saplings which have not developed the corky bark of a mature Oak, are fair game.
Mine love the bark of Ash, not sure about the leaves.
No experience of Sycamore