o'callaghan's career in radio started in 1951 when he joined 2sm as an office boy, aged 17. he did his first on-air work within a few weeks. this began what the "daily telegraph" newspaper called "one of the most distinguished careers in the history of australian broadcasting". early in his career, in the 1950s, he was notable for his exclusive coverage of the cold war defection that became known as the petrov affair. he joined 2ue in 1956, where he was known for his long running, and ratings winning, breakfast show. he stayed with 2ue until retirement in 2003, then had a short time at 2ky during kerry packer's ownership of 2ue. he moved back to the mid north coast of nsw where he continued to work at 2mc.