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
A pledge is basically very serious formal promise. You can pledge allegiance to your country, you can pledge to keep a secret, and you can pledge a sum of money to a cause. Pledge can be used as both a noun and a verb. As a noun, it can be a solemn promise you've made. Or even the person who makes that promise, like the freshman pledges who take an oath to join a fraternity in college. As a verb, it describes the act of promising.