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Where did the Old Testament teach, “By the OT alone!”

Bind up the testimony, seal the law among my disciples. (Isa 8:16) To the law and to the testimony: if they speak not according to this word, it is because there is no light in them. (Isaiah 8:20)

Thus the Lord invoked the tripartite canon of the Law, the Prophets and The Writings in epistemologically establishing His messiahship and ministry and opened the minds of the disciples to the same, who did the same. (Luke 24:27.44,45; Acts 17:2; 18:28, etc.)

While more wholly God-inspired words would be added, these had to be in conformity with those which were already established as being so, which itself set the standard for a true prophet. (Deuteronomy 18:21,22)

And thus it was not promulgation of some oral tradition that Ps 19 and Ps. 119 exalts, nor which resulted in great revival, but as in

"when they brought out the money that was brought into the house of the Lord, Hilkiah the priest found a book of the law of the Lord given by Moses. And Hilkiah answered and said to Shaphan the scribe, I have found the book of the law in the house of the Lord. And Hilkiah delivered the book to Shaphan. (2 Chronicles 34:14-15)

Then Shaphan the scribe told the king, saying, Hilkiah the priest hath given me a book. And Shaphan read it before the king. And it came to pass, when the king had heard the words of the law, that he rent his clothes. And the king commanded Hilkiah, and Ahikam the son of Shaphan, and Abdon the son of Micah, and Shaphan the scribe, and Asaiah a servant of the king’s, saying, Go, enquire of the Lord for me, and for them that are left in Israel and in Judah, concerning the words of the book that is found: for great is the wrath of the Lord that is poured out upon us, because our fathers have not kept the word of the Lord, to do after all that is written in this book. "(2 Chronicles 34:18-21)

The prophet here spoke as wholly inspired of God but in confirmation of obedience to the Law. But unless you want to assert that your past RC magisterium spoke as wholly inspired of God as the writers of Scripture did, then you are stuck with them having non-infallible authority, by which they can even claim to "remember" what ancient history forgot, as in the case of the Assumption.

As Ratzinger states,

Before Mary's bodily Assumption into heaven was defined, all theological faculties in the world were consulted for their opinion. Our teachers' answer was emphatically negative . What here became evident was the one-sidedness, not only of the historical, but of the historicist method in theology. “Tradition” was identified with what could be proved on the basis of texts. Altaner , the patrologist from Wurzburg…had proven in a scientifically persuasive manner that the doctrine of Mary’s bodily Assumption into heaven was unknown before the 5C ; this doctrine, therefore, he argued, could not belong to the “apostolic tradition. And this was his conclusion, which my teachers at Munich shared .

This argument is compelling if you understand “tradition” strictly as the handing down of fixed formulas and texts [meaning having actual substance in history]…But if you conceive of “tradition” as the living process whereby the Holy Spirit introduces us to the fullness of truth and teaches us how to understand what previously we could still not grasp (cf. Jn 16:12-13), then subsequent “remembering” (cf. Jn 16:4, for instance) can come to recognize what it has not caught sight of [even bcz there was nothing to see] previously and was already handed down [invisibly, without evidence] in the original Word,” — J. Ratzinger, Milestones (Ignatius, n.d.), 58-59 (words in [brackets] are mine).