Yes, oak leaves are rich in tannins. That's why oak bark, and leaves, were used to tan leather, but they still compost. However, to compost, you need air, water, sources of carbon (e.g. leaves), and, ideally, nitrogen. The higher the carbon to nitrogen ratio is, the longer it will take to decay into compost. You have to feed the fungi that will convert the leaves (carbon). Rain by virtue of falling through the air does absorb some nitrogen (ammonia and nitrogen oxides) from the air and will contribute a little. Mixing in something like urea or chicken manure will really accelerate the decomposition.
The other trouble with oak leaves is that they are really flat, so they pack down, which shuts off the air supply, slowing the composting process. Turning the compost helps aerate things, makes new air tunnels, and helps redistribute nutrients.
I hope that this helps.
All the best,
Peter