I have 2 nuttal oak trees I planted about 2 years ago, 5 gallon containers. I noticed today that 1 already has reddish leaves and one is green. When I looked closer at the "red" one, I noticed that the bark on the lower side of the tree is coming off. (I covered this area with tree-kote last year because I noticed a deer had been rubbing it.) What can I do to keep this tree from dying?

Staciec, aside from the bark which squirrels and deer alike do make a meal of, acorns produced by the tree will also be an invite.

The only way you can be sure to prevent such removal of the bark is to put a sturdy, long lasting fence in its path.
A fence built outward from the trunk of chicken wire--such that the squirrel or deer cant get to the bark may be the only way.

Since the deer cant be thought to be prevented from getting to the tree, you may have to throw up your hands and admit defeat...he will get his way.

There are pest repellants available at most garden centres. One is called "Ropel"...its sprayed on the surface of the tree and probably will have to be re-applied when rain beats it.

You might also keep snow cover from piling up at the base of the tree. This can often make it easier for pests to climb to the wood.

Using 4 inch flexible and perforated weeping tile cut to length and split down the middle so you can open it up and place around the trunk will help protect the trunk from critters.

As far as keeping the tree from dying and bring it back to health, there are professional and experienced tree people here to give you help. Give your post a day or two for them to see this.