All throughout the OT, we see the direction to those out of fellowship to return to God so He can return to them. Repentance isn’t unique to NT theology.
Good, but who does the calling to repentance and by what authority? Also, why was John baptizing and by what authority? Plus, there is the tiny problem of those out of fellowship being the entire world, not just Israel. How are other nations and peoples called to repentance during the OT? NT? Are they lost forever?
Repentance isn’t unique to NT theology.
Correct and obedience isn't just a OT theme either. Works are a fundamental part of Christian success and the NT points that out fairly clearly.