The oath is in Article II, §1, Clause 8, of the #US #Constitution, & a new #president must take it before exercising or carrying out any official powers or duties:
Before he enter on the Execution of his Office, he shall take the following Oath or Affirmation— "I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, & will to the best of my ability, preserve, protect & defend the Constitution of the United States."
https://www.law.cornell.edu/constitution-conan/article-2/section-1/clause-8/presidential-oaths-effect-on-executive-power