Learner's definition of OATH
a formal and serious promise to tell the truth or to do something
They were required to take/swear an oath of loyalty. [=promise formally to remain loyal]
an oath to defend the nation
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]
an offensive or rude word that is used to express anger, frustration, surprise, etc.
having made a formal promise to tell the truth in a court of 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.”
He said in testimony given under oath that he was not there the night of the crime.