Terri and Peter recently bought a used solid oak French door, but it has a warp in it. How to take it out?
Bending wood (or unbending wood) is usually done by getting moisture into the wood, forcing it into a shape and then letting it dry under force. For serious wood bends we use steam chambers. For a door like this, I would suggest first stripping the door so that moisture can get through the finish.
Since a steam chamber large enough for the door is probably impractical in a basement shop, I would suggest simply putting down a plastic sheet, then wet towels, then the door, then more wet towels, then another plastic sheet. The idea is to trap the moisture in between the two sheets of plastic and force it to go into the wood of the door. Leave it like this for a day or two. You could even rewet the towels if they dry out.
Then put a spacer in the middle, and a 2x4 across the warp. Use two clamps to force the door into the opposite warp. You want to force it as far in the other direction as the warp was in the first place, because when you take off the clamps, it will bounce back a bit. Leave it for another couple of days. Now remove all the towels and plastic, but put the clamps back on while it dries, again bent slightly in the opposite direction from the original warp. When it is completely dry, release the clamps and see what you have accomplished.
This is definitely a trial-and-error procedure. You could go too far, or not far enough. But with enough moisture, and enough patience, you can change the shape of any wood. Once you get it where you want it (judging it when it is free of clamps and dry), immediately put a sealer on the door, including a generous amount on all end grain.
Oh, check out the really cool clamps from American Tool Co. in the photo. They have both the quick grip and the power screw-down.