[akhtene]

I would think it depends on TO whom and BY WHOSE NAME(S) the oath is sworn.

In Feanor’s case the oath was sworn to themselves, nobody really wanted or expected them to do what they promised. So in case the oath givers \ takers settled the matter among themselves, the Valar (as witnesses to it and the Supreme Court in one) would decide whether they had the right to do it, as well as to start the whole affair.