Tortoise safe or no? I live in south Texas and oak trees are EVERYWHERE. The leaves fall into my yard from the other side of the fence. They will inevitably be within reach of my leopards when they are outside, when they are a bit bigger.
The leaves are toxic to mammals, so probably not real good for tortoises either, but I'd be willing to bet that if there is other good graze in the pasture he might try a leaf, then quickly decide he doesn't want to try any more.
Oaks are high in tannins, this is a chemical that blocks digestive enzymes and, at high levels, can be toxic.
I'd be willing to bet your torts will try them and give 'em up pretty quick, I wouldn't worry too much about it. Certainly don't feed them the foliage but don't go cutting all your oaks down.
If they eat the green leaves, they are toxic. The old dried up ones are supposedly okay though. I doubt they will eat them. I know this because many roach keepers use old brown oak leaves in their set up for the roaches to chew on.