With oak, you want to use the sapwood instead of the heartwood for the back of the bow. Just peel the bark off the stave and use that as the back of the bow, that way you have a continuous unbroken growth ring for the bow back. If you're using heartwood you have to work it down to a single growth ring from one end to the other, or there is a good chance that the bow will break. The only common woods that the heartwood is normally used on are osage orange, black locust, and mulberry. Yew bows are often made from half heartwood, half sapwood. Also, you don't want to trim a stave down and bend it too far until it is tillered to bend evenly. If there is a spot where the bend is uneven, it will usually fail or crack there. I recommend that you get The Traditional Bowyer's Bible vol. 1 and read through it, lots of good info in it.