An oath is a promise. It’s usually part of a formal ceremony, but it can be informal as well. The word "oath" is a noun. It’s often used with the verb "take."
During a wedding ceremony, a man and a woman take an oath. They promise to always be faithful and supportive to each other.
People in a court of law take an oath and promise to tell the truth.
If you don’t tell the truth in a court of law, a judge will remind you that you are "under oath." If you lie, you could go to jail.
a judge
During an inauguration, The President of the United States takes an oath to support and defend the Constitution of the United States.
Police officers take an oath to uphold the law.
Medical doctors take the Hippocratic Oath and promise to do the best they can for their patients.
