That's the performance of the oath (or "promise") made to Abraham, not the oath, per se. Read it, it's right there in the text:Yes it was. It was the reason why Abraham was blessed, because of his righteousness:
Genesis 26:3 Sojourn in this land, and I will be with thee, and will bless thee; for unto thee, and unto thy seed, I will give all these countries, and I will perform the oath which I sware unto Abraham thy father;
4 And I will make thy seed to multiply as the stars of heaven, and will give unto thy seed all these countries; and in thy seed shall all the nations of the earth be blessed;
5 Because that Abraham obeyed my voice, and kept my charge, my commandments, my statutes, and my laws.
Gen 26:3 Sojourn in this land, and I will be with thee, and will bless thee; for unto thee, and unto thy seed, I will give all these countries, and I will perform the oath which I sware unto Abraham thy father;
It's the same thing with justification that Paul explained in Rom 4:5. The objective fact of our justification being declarative and God imputing (performing) that declaration (the "promise") to us subjectively by/through our faith therein.