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.

Some oaths of office are statements of allegiance and loyalty to a constitution or other legal text or to a person or office-holder (e.g., an oath to support the constitution of the state, or of loyalty to the king or queen) (see Oath of allegiance). Under the laws of a state, it may be considered treason or a high crime to betray a sworn oath of office.

The word "oath" and the phrase "I swear" refer to a solemn vow. For those who choose not to, the alternative terms "solemn promise" or "solemnly affirm" and "I promise" or "I affirm" are sometimes used.