This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong; it cannot perjure itself; it cannot be wholly absent. Only its interpretation can err.
Oath definition, a solemn appeal to a deity, or to some revered person or thing, to witness one's determination to speak the truth, to keep a promise, etc.: to testify upon oath. See more. Dictionary.com
The essence of a divine oath is an invocation of divine agency to be a guarantor of the oath taker's own honesty and integrity in the matter under question. By implication, this invokes divine displeasure if the oath taker fails in their sworn duties.