'Oath' in the Bible

All the people came to urge David to eat food while it was still day; but David took an oath, saying, “May God do so to me, and more also, if I taste bread or anything else before the sun sets.”

Therefore the king said to Shimei, “You shall not be put to death.” And so the king gave him his promise.

But the king spared Mephibosheth the son of Jonathan, the son of Saul, because of the Lord’s oath that was between David and Saul’s son Jonathan.