The answer to your question depends on who you are asking about.
Since you didn't say what oath I will answer the question in a general way. Most oaths ask a person to follow the law and have them,promise to uphold the constitution.
While taking Oath as newly inducted citizens of USA, one has to promise to bear arms in defense of the country if time comes. This promise is unique to USA oath.
promise
I swore an oath to always uphold the law. (Make sure to keep the promise) Heed my advice and you will succeed. (Pay attention to what I say)
pledge, promise, swear
promise
pledge
what is another word for promise? A vow or an oath
promisedguaranteedsworevowed
If you're inquiring as to what "oath" means in terms of speaking "under oath," then the following (from wikipedia) is likely an adequate answer: An oath (from Anglo-Saxon āð) is either a promise or a statement of fact calling upon something or someone that the oath maker considers sacred, usually a god, as a witness to the binding nature of the promise or the truth of the statement of fact. To swear is to take an oath. Or simply stated, it is a promise to tell the truth.
The president promise to defend the Constitution.