I have been growing some indoor plants as a project with my son, including a small oak sapling that grew in a planter we had outside during the summer. One of our cats, for reasons unknown, ate the top of the sapling and all but one of the leaves. The oak now maintains that leaf, but all other growth has stopped (~1wk); can anything be done to stimulate upward growth and more leaves? The plant is very young and only ~4" high.
Oaks are deciduous trees, so it shouldn't really be growing (or have any leaves) at this time of the year anyway, assuming you are in the northern hemisphere.
They are also completely hardy, so there was no particular reason to bring it inside for the winter!
Small trees usually survive being trodden on or eaten, if they have enough roots. A tree grown in a pot may have its root system restricted. In the first year, trees growing in the ground often produce more roots than branches and leaves. A four inch tall tree may have a tap root going down a foot or more.
Don't expect it to grow new leaves for several months. Oak trees growing outside in the UK don't come into leaf till about the end of April. Just put it somewhere cool but with good light, give it very little water since it's not going to grow much right now, and wait.
The "somewhere cool" is important. The dormant buds on a oak will not start to develop until the tree has had a sufficient length of time at temperatures below 50F (10C). Keeping it indoors "because it is only tiny" may be the worst thing you can do with it.
There isn't much you can do except wait; fertilizing now isn't a good idea, so just give it time to see if it continues to grow and puts out new leaves. In regard to your cat, if its a cat that doesn't have access to outdoors, or outside is all snow, the cat probably needs some cat grass indoors to nibble on. Unless its a kitten, cats usually only eat plants indoors if they're desperate and don't have access to grass.