Complex sentence: Oppenheim directed the Masterworks division of Columbia Records from 1950 to 1959, during which he worked with numerous notable figures in the music world including Igor Stravinsky, whom he formed a friendship with, later producing for him.
Split into simpler sentences: Oppenheim directed the Masterworks division of Columbia Records from 1950 to 1959. During this time he worked with numerous notable figures in the music world including Igor Stravinsky, with whom he formed a friendship, later producing for him.