Usually oak leaves and their fruits (acorns) contain copious amounts of tannins, which inhibit digestion. When you see bears and deer eating acorns, usually their salivating heavily as a response to the tannins and other toxins within the nuts (salivary amylase and other proteins/enzymes bind to the tannins in the acorns negating the digestion inhibition).
From what I've read here and elsewhere, tortoises generally shouldn't eat nuts and grains. I'd say if there's a portion of the yard that receives less acorns or if there is some way to keep acorns from falling within their enclosure, you're all set. Otherwise, I probably wouldn't entice them to try any by placing an enclosure directly beneath an oak. You have to be careful of the leaves too, they're really heavy in secondary defense compounds.
Acorns, oak leaves, etc. are all on the toxic list and are some of the rather small number of things KNOWN to be dangerous to reptiles.
This does not mean they would try to eat them, however. That is a completely different question that I don't have a good answer to- although there are records of it happening at least sometimes.
I have an oak tree in my backyard too. And when I let my Red Food loose in the backyard he doesn't pay any attention to the acorns. So I agree with the previous post. Everything will be fine
I think your tortoise will taste the tannins long before he eats dangerous quantities. Redfoots probably don't encounter oaks in their natural habitat, but I believe oaks are an "obvious" hazard to a tortoise. My Greeks have 4 different species of oaks in their pen. Sometimes I notice a bite taken out of a leaf but it's always just one.