And no, oaths are most emphatically not
magical. If you're a witness in court in the UK and you swear on the Bible to tell "the truth, the whole truth, and nothing but the truth, so help me God" that's an oath. The whole point about such things is that to be forsworn invites divine punishment. That's the sort of thing oaths have always been about - swear by the gods, and the gods will be displeased or angered if you break your word, and Bad Things will happen. Curses are similar - if you say "God damn you" then that's a curse, you're asking God to punish someone using his power. Nothing to do with you at all, if you truly mean it you're really hoping he'll hear you. Same thing with Isildur - he curses those Men, his words are heeded, and *pow* divine punishment.
So no, I don't think you understand oaths, and if you don't understand them then I'm afraid you don't understand the Sil, either. Which explains a lot.