Born in Dublin, he successfully auditioned for Radio Éireann when he was 18 and gave his first commentary on 14 August 1938, when Galway defeated Monaghan in the All-Ireland Football Semi-Final.
His voice was to become synonymous with radio coverage of hurling and football, and his broadcasts were important to the thousands of people who gathered around radio sets in the 1940s and 1950s in Ireland and abroad. He covered major GAA matches from 1938 until 1985, when illness prevented him from covering his 100th All-Ireland final.
O'Hehir became racing correspondent for the 'Irish Independent' in 1947, and went on to work for the BBC and ABC in America. He was appointed Head of Sports Programmes when RTÉ established a television service in 1961.
His skills as a commentator took him in a different direction in November 1963. A trip to New York with his wife coincided with the assassination of U.S. President John F. Kennedy. The responsibility to comment on the funeral on behalf of Telefís Éireann fell to O'Hehir and his coverage won him praise in both Ireland and the US. He later became manager of Leopardstown Racecourse (1972-73).