Oaktree Capital Group LLC co-founder Howard Marks is known as a shrewd investor. One of his best decisions was spending about $20 million in 2009 for a one-fifth stake in Jeffrey Gundlach’s upstart investment firm. Gundlach’s DoubleLine Capital has paid out $158 million in income to Oaktree over the last five years, repaying that original investment almost eightfold. What’s more, the stake may now be worth as much as $900 million,...