What is the ruling on one who swore an oath to stop touching pimples, then returned to doing so? Is expiation required of him? And is his action considered unlawful as a form of self-harm, knowing that he finds comfort in it and it might be a type of obsessive-compulsive disorder?

The phrase "I covenant with Allah to do such-and-such" constitutes an oath and a vow if one commits to it as an act of drawing closer to Allah (qurbah) and obedience (ta'ah). It is considered an oath only if one commits to it in matters that are not acts of drawing closer to Allah. Whatever is a vow must be fulfilled. Whatever is an oath, one has the choice between proceeding with it or offering expiation (kaffarah). It is not permissible to harm oneself unless the questioner is overwhelmed by her circumstances.