An Australian international and New South Wales interstate representative prop forward, he played in the New South Wales Rugby League premiership, mainly for the Parramatta Eels, but also for the Eastern Suburbs Roosters and Penrith Panthers. Nicknamed 'The Bear', O'Reilly enjoyed a fifteen year Sydney first grade career debuting for Parramatta at aged eighteen in 1967 and returning to the club in 1981 to first taste premiership success. He represented Australia in nine Tests and seven World Cup matches, including the Kangaroos' victory in the 1970 World Cup.
He was the first Parramatta junior to represent Australia, having played his junior football with suburban Guildford. O'Reilly is named on the Australian Players Register as Kangaroo No. 450. A decade after his first representative appearance, O'Reilly was part of Parramatta's first ever premiership-winning team of 1981.
He held the record for highest number of first grade games at 284 until overtaken by Geoff Gerard. Bob O'Reilly was awarded Life Membership of the Parramatta Eels Club in 1981.