Have you taken all of the back off, down to sap wood? An efficient way to dry out oak quickly is to leave its cambium on, at let it sit somewhere hot and dry for a while. The inner bark will help regulate the loss of moisture and keep it even and slow over the surface area of the wood.
The biggest oak club I've ever worked was once a 25 year old white oak tree. Dug it up by the root and left it to sit in the garage for 6 months with ALL bark on. Then I took it down to its cambium layer and left it to set for another 6 months. After that, removed all traces of bark, and let sit for another whole year. This was a big guy though, 6 inch wide head, 2.5 inch trunk (eventually slimmed down). The drying time goes up exponentially as the club gets thicker. If you really like the piece, take it slow. I hade a hickory club that I moved too quickly with and it cracked too much to continue.
Don't bother with the whole 'cover in butter and put up chimney' thing. I did however put my clubs in an oil bath (usually just vegetable oil) after finishing them with a nice oil stain. The oil fills in the spaces once filled with water, gives the club its resilience.
John