Glasgow born inside forward Paddy O’Brien started his career playing junior football in the Glasgow, playing for Maryhill Myrtle in 1891, Elm Park in 1892 and Glasgow Northern in 1893, before making the move south to join Second Division Woolwich Arsenal in April 1894. He made his Football League debut against Grimsby Town on 10th September 1894 and went on to score eleven goals in 28 games for Woolwich Arsenal in his first season as Woolwich Arsenal finished 8th in the Second Division.
An injury at the start of the 1895-96 season ruled O’Brien out for four months, hampering his goal tally. However, he came back the next season, 1896-97, scoring 15 goals in 28 appearances, making him the club’s top scorer that season. In total he played 67 times for Arsenal, scoring 29 goals. However, despite his good goalscoring record, he was sold to Southern League Bristol City in the summer of 1897, linking up with former Arsenal boss Sam Hollis, who had also just moved there. In 1897-98 and 1898-99 The Robins finished as runners up in The Southern League.
He spent five seasons with Bristol City, including scoring 7 goals in 8 League games during their inaugural Football League season in 1901-02 before seeing out his career with Southern League Swindon Town, making a single appearance for the Robins in 1902-03, playing against Kettering Town in December 1902.