Howard Marks is a study in contrasts. On the one hand, he has the undeniable mien of an academic, with his clear horn-rim glasses, conservative dress, and close-cropped, spiked hairstyle. His manner during a long interview with Barron's is both diffident and didactic, though a certain intensity crackles just beneath the surface.

Yet, over his four decades on Wall Street, the trim 67-year-old co-founder and chairman of Oaktree Capital Management has acquired true star power -- partly because the size...