A left half, O'Farrell started his playing career with League of Ireland club Cork United, replacing Tommy Moroney in the first team after his departure to West Ham United in 1947. Playing as a semi-professional, he supplemented his earnings with his work on the railways, earning £3 a week for each. In January 1948, after being spotted by West Ham scout Ben Ives, O'Farrell followed Moroney to the Upton Park club for a fee of £3,000. O'Farrell himself received a £1,000 fee for the transfer. He made over 50 appearances for the reserves before breaking into the first team. His debut came on 28 September 1950 in a 2–0 defeat of Colchester United in the Essex Professional Cup. His Football League debut followed in November 1950 in a 4–1 defeat away to Notts County. He made 18 league appearances in his first season but became a regular in the 1951–52 and 1952–53 seasons. He was one of a number of West Ham players that would meet at Cassettari's Café to discuss tactics. He played for West Ham in seven seasons and made 213 appearances, scoring eight goals.