I'm aware that by tradition a President is sworn in by a SCOTUS justice, but I'm wondering if this is necessary? My context is the show Designated Survivor. Absent SCOTUS, what if any provision is made for which judge or authority may administer, witness etc the oath of office?
Article II Section 1 Clause 8 of the United States Constitution simply states that the President has to take an oath on the execution of his office.
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, and will to the best of my ability, preserve, protect and defend the Constitution of the United States."
The constitution does not mandate that the oath must be administered by a SCOTUS justice even though it has been the norm for the Chief Justice of the Supreme Court to do so.
In fact, some Presidents were sworn in by a US District Court judge. The most recent case being the first inauguration of Lyndon B. Johnson. Johnson was sworn in by Sarah T. Hughes, a judge on the US District Court for the Northern District of Texas. Wikipedia has a list of swearing-in ceremonies.
Anyone can swear in the President, though you are correct that traditionally that role has fallen to the Chief Justice of the Supreme Court.
There's a good Wikipedia article for this question which contains the following excerpt:
George Washington was sworn into office during his first inauguration, on April 30, 1789, by Chancellor of New York Robert Livingston. William Cranch, chief judge of the U.S. Circuit Court, administered the oath to Millard Fillmore on July 10, 1850, when he became president after the death of Zachary Taylor. Upon being informed of Warren Harding's death, while visiting his family home in Plymouth Notch, Vermont, Calvin Coolidge was sworn in as president by his father, John Calvin Coolidge Sr., a notary public. Federal Judge Sarah T. Hughes administered the oath of office to Lyndon B. Johnson aboard Air Force One after John F. Kennedy's assassination on November 22, 1963. This was the first (and to date only) time a woman administered the oath of office. Overall, the presidential oath has been administered by 15 Chief Justices (one of whom—William Howard Taft—was also a former president), one Associate Justice, four federal judges, two New York state judges, and one notary public.