Most translations say something like "And how can they believe in the one of whom they have not heard?" making οὗ indirect, someone heard about or heard of from a third party. But the New American versions make οὗ the direct object of ἤκουσαν, the speaker who is heard. Michael Horton's systematic theology makes a point of the latter; Michael Bird (who wrote a Romans commentary) understands it to be the former.Πῶς οὖν ἐπικαλέσωνται εἰς ὃν οὐκ ἐπίστευσαν;
πῶς δὲ πιστεύσωσιν οὗ οὐκ ἤκουσαν;
πῶς δὲ ⸀ἀκούσωσιν χωρὶς κηρύσσοντος;
I understand both options to be legitimate; the former seems to fit the context better, giving οὗ the same role as ὃν, and focusing on the agency of the messenger, but the latter is more powerful rhetorically, linking believing/trusting in Christ with a direct word from Christ, which seems to be in view in v. 17 (ἄρα ἡ πίστις ἐξ ἀκοῆς, ἡ δὲ ἀκοὴ διὰ ῥήματος ⸀Χριστοῦ.).
How should we understand this word οὗ, and why? Are there considerations I'm missing that decide the issue?