Dan Och still runs the show at Och-Ziff Capital Management Group. He’s the chief executive officer, the chairman of the board and very much the face of the hedge-fund powerhouse he founded more than two decades ago. But in the wake of a bribery scandal that spooked clients and blew away a third of its assets, the fund’s fate is in many ways now in the hands of a little-known 34-year-old named Jimmy Levin. Back in February, Och shocked many on Wall Street by elevating Levin, the star of the firm’s credit business, to co-chief investment officer and handing him...