He (Oak) is fairly well-known after Hamilton.
Oh you're funny. Hardly anyone outside of absolute Hamilton fandom knows him - it's a big fandom, but they are more likely to follow Phillipa Soo to watch Amelie than see Oak in a replacement role.
And yes, Josh isn't selling ALL the tickets, but he really is selling a significant portion of it. Something which will become extremely apparent come fall.
Caption: Every so often there was a rare moment of perfect balance when I soared above him.