Oil stones are known for their durability, and many are actually passed down from father to son. Eventually, even the well-maintained oil sharpening stone will become dished, usually in the middle, from the effects of repeated sharpening over many years. Once this happens, the stone will no longer be able to sharpen properly and you will have to level, or flatten, it to bring it back to usability.
Spray the back of the 220 grit sandpaper and stick it with the grit side up on a flat surface.
Apply honing oil to the top of the oil stone.
Flatten the stone by rubbing the face of the sharpening stone on the sandpaper in a circular or figure eight motion, stopping periodically to check the face for flatness. Stop rubbing it once the face is flat.
Repeat the process using progressively finer grades of sandpaper, if needed, to remove scratches left by the previous grit.