an 'oath of office' [=an official promise by a person who has been elected to a public office to fulfill the duties of the office according to the law]
In a U.S. court of law, a witness must swear 'under oath' to tell “the truth, the whole truth, and nothing but the truth.”
They were required to take/swear an 'oath' of loyalty. [=promise formally to remain loyal]
(..) you couldn't find him. That's what it means to lose something. Now he's come back and taken Riverrun. I don't think it's fair to blame us... For years we kissed Tully boots, swore oaths to them and their stinking fish banners. Not again. Riverrun is ours. Take it back! We don't have the men. (..)2016 TvShow: Game of Thrones Title: Blood of My Blood Season: 6 Episode 6