"As the Lord liveth" was the most common form of oath among the Israelites (e. g. Judges 8:19; 1-Samuel 14:39; 1-Samuel 19:6). It was unique to David to attach a further clause to this oath - a clause of thankfulness for some special mercy 1-Samuel 25:34, or for God's constant protection of him (here and in 2-Samuel 4:9).

And the king sware,.... To his former oath, he added another for greater confirmation:
and said, as the Lord liveth; which was the proper form of an oath, which ought to be taken by the living God; and as what would lay him under the greater obligation to observe it, he adds,
that hath redeemed my soul out of all distress; saved his life when in the most imminent danger; delivered him out of the hand of Goliath, and from the Philistines and other enemies, in his wars with them; and from Saul and his persecuting rage and fury, and from the rebellion of his son Absalom, and the insurrection of Sheba.

Out of all distress - The words contain a grateful acknowledgement of the goodness of God to him, in bringing him safe through the many difficulties, which had lain in his way, and which he now mentions to the glory of God, (as Jacob when he lay a dying) thus setting to his seal, from his own experience that the Lord redeemeth the souls of his servants.