'Oath' in the Bible

For men swear by what is greater than themselves; and with them an oath in confirmation of a statement always puts an end to a dispute.

He added an oath, in order that, through two unchangeable things, in which it is impossible for Him to prove false, we may possess mighty encouragement--we who, for safety, have hastened to lay hold of the hope set before us.

And since it was not without an oath being taken--

for these men hold office without any oath having been taken, but He holds it attested by an oath from Him who said to Him, "The Lord has sworn and will not recall His words, Thou art a Priest for ever" --

For the Law constitutes men High Priests--men with all their infirmity--but the utterance of the oath, which came later than the Law, constitutes High Priest a Son who has been made for ever perfect.