We need both the Promise and the Oath:
The promise discloses the mind and intent of God.
....God’s promise is found in His Word. He gave us His word so that when we speak God’s word out of our mouth, it has the same integrity and same power as when He originally spoke it.
....God’s mind and thoughts are revealed in what He’s promised us.
In the World there is a resistance against the Word of God. This world’s system fights the Word, so it is a fight of faith to believe and hold fast to the Word. That is why God also gave us an oath. The oath discloses the character of God. The Oath lets us how strongly God feels about what He will do for us.
...Hebrews 6:13 - For when God made a promise to Abraham, because He could swear by no one greater, He swore by Himself, saying, “Surely blessing I will bless you, and multiplying I will multiply you.
His oath brings us STRONG CONSOLATION so we can lay hold of the hope (promise) He’s set before us.
God’s Oath exhibits his extreme desire to