O'Neill is a guitarist who played professionally and joined the orchestras for Broadway productions of Hair and Jesus Christ Superstar. His early musical influences were the rock groups Queen and Yes, and performers Jim Croce and Harry Chapin. In the mid-'80s he began working as a producer with groups including Aerosmith and Badlands. He became a concert organizer and producer, and was invited to put together some of the first major rock concerts held in Japan. This gained him understanding of Japanese working conditions and audience preferences, which led him to be engaged to organize tour appearances in Japan for major rock acts such as Madonna, Sting, and Bon Jovi. He wrote two complete rock operas, Streets and Dead Winter Dead.
He conceived a rock album, Christmas Eve and Other Stories, focusing on the theme that small acts of generosity or honesty can have far-reaching positive impact (the message of Frank Capra's film It's a Wonderful Life). For it, he planned the sound of the large symphonic rock orchestra, and named it The Trans-Siberian Orchestra, though neither it nor O'Neill have any connection with Siberia.