Here "the law" is a metonym for the men who appointed the high priests according to the law of Moses. The focus is not on the men who did this, but on the fact that they did this according to the law. Alternate translation: "according to the law, men appoint as high priests men who have weaknesses" or "for according to the law, men who have weaknesses are appointed as high priests" (See: [[rc://en/ta/man/translate/figs-metonymy]])
The "word of the oath" represents God who made the oath. Alternate translation: "God appointed a Son by his oath, which he made after he gave the law" or "after he had given the law, God swore an oath and appointed his Son" (See: [[rc://en/ta/man/translate/figs-metonymy]])
This can be stated in active form. Alternate translation: "who has completely obeyed God and become mature" (See: [[rc://en/ta/man/translate/figs-activepassive]])