Let oak season long enough to let the tanins(sp?) dry and leach out. It is lethal. Test it by putting a piece of oak in non-chlorinated water. If the water changes color in 5 or 6 days, don't use it. After about a week, put a couple of small fish in to see if they live. Bass seem to prefer 'fluffy' structure. I guess it is because they are essentially ambush predators.