An oath of office is an oath or affirmation a person takes before assuming the duties of an office, usually a position in government or within a religious body, although such oaths are sometimes required of officers of other organizations. Such oaths are often required by the laws of the state, religious body, or other organization before the person may actually exercise the powers of the office or organization. It may be administered at an inauguration, coronation, enthronement, or other ceremony connected with the taking up of office itself, or it may be administered privately. In some cases it may be administered privately and then repeated during a public ceremony.

Traditionally an oath is either a statement of fact or a promise taken by a sacrality as a sign of verity. A common legal substitute for those who conscientiously object to making sacred oaths is to give an affirmation instead. Nowadays, even when there is no notion of sanctity involved, certain promises said out loud in ceremonial or juridical purpose are referred to as oaths. "To swear" is a verb used to describe the taking of an oath, to making a solemn vow.