Complex sentence: When he was 17, Oskar decided that the United States was where a harmonica player should make his career, so he moved to New York at the age of 18 with little more than a harmonica in his pocket.
Split into simpler sentences: When he was 17, Oskar decided that the United States was where a harmonica player should make his career. So he moved to New York at the age of 18 with little more than a harmonica in his pocket.