We know the promise precedes the law, but Hebrews 7 tells us that the oath proceeds it.
The oath as in Heb 6, is to confirm the promise.
Thus the law comes in between the promised work of Christ and the oath which cements His work into eternity, given after Jesus sat down. The law is but a temporary blip between 2 unchangeable things.
It is then to our comfort that the oath confirms that Jesus blood once shed for all continues to be eternally effective for us. His priestly work continues till this very day.
"I have sworn by myself you are a priest forever..."