oath plural oaths [countable]
a formal and very serious promise
oath of loyalty/allegiance/obedience etc (to somebody)
an oath of allegiance to the Queen
swear/take an oath
Servicemen have to swear an oath of loyalty to their country.
The president took the oath of office (=promised to do a government job well before beginning it).
a formal promise to tell the truth in a court of law
The evidence was given under oath.
Witnesses are required to take the oath (=make this promise).
an offensive word or phrase that expresses anger, surprise, shock etc:
He was shouting out oaths as they led him away.