I've never heard of un-warping a white oak jo, but maybe it can be done. The Kingfisher site has great advice on avoiding warping in the first place. In addition to what they say, I have been advised not to store a jo in an upright position or on those two-points-of-contact weapons racks that are so popular. My jo lie flat on the floor whenever I'm not using them. The one jo that I foolishly left leaning against the wall in a corner warped -- another one that I have never stored improperly is still straight as a ruler after 15 years.