a promise that that a person will tell the truth (especially in a court of law), especially one that makes reference to God or to a holy book I must remind you that you are under oath. Think very carefully before answering the question.
- She swore on oath that it was not true.
- The witness refused to testify (=give evidence in court) under oath.
- It is time for the new president to take the oath of office.
- Under oath is used in AmE and BrE, whereas on oath is only a BrE usage.