- OpenID is (mainly) for identification/authentication, so that
stackoverflow.comknows that I ownchris.boyle.name(or wherever) and therefore that I am probably the same person who ownedchris.boyle.nameyesterday and earned some reputation points. - OAuth is designed for authorization to take actions on your behalf, so that
stackoverflow.com(or wherever) can ask permission to, say, Tweet on your behalf automatically, without knowing your Twitter password.