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]
(..) To Sansa Stark. Who are you? Someone who swore to keep her safe. Swore to who? Her mother. Her mother's dead. That doesn't release me from an oath. I served Lady Catelyn. I serve her still. Who do you serve? Myranda. I saw you staring at her. I'm going to marry her. That will involve looking (..)2015 TvShow: Game of Thrones Title: Kill the Boy Season: 5 Episode 5
(..) Because you are not my enemy. And I'm not yours. You sure seemed like my enemy when you were killing my friends. For years the Night's Watch have sworn an oath to be the shield that guards the realms of men. And for years we've fallen short of that oath. You belong to the realms of men. (..)2015 TvShow: Game of Thrones Title: Kill the Boy Season: 5 Episode 5