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. AT: "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. AT: "by his oath, which God made after he gave the law, God appointed a Son" or "after he had given the law, God swore an oath and appointed his Son" (See: [[rc://en/ta/man/translate/figs-metonymy]])