Whether you are trying to salvage a stiff brush or dried out paints, or redo a mistake on a dried painting, there is one quick and easy to use method to soften oil paint. Keep in mind, however, that turpentine can be toxic if inhaled, so only use it in a well-ventilated space.
Acquire turpentine. This can be easily found in most art or paint supply shops. It is also used with oil-based house paints.
Assemble your painting, brush or squeeze out some dry paint from an old tube.
Apply turpentine as necessary. For a painting or a large space, put on gloves and apply turpentine to a rag. Place this rag on the surface and let it soak in for a few minutes. Use more as necessary until the paint softens. To soften a brush, pour a small amount (enough to cover all the bristles) in a container and leave the brush in it until it softens. You cannot use this container for eating or drinking after it has been used for turpentine. To fix small detailed work or to remove paint dregs from a tube, soak a brush in turpentine and run it over the paint until it softens and becomes workable.