Alex Ferguson took over from Atkinson as manager in November 1986, and O'Brien made his debut in the 1986–87 season, making nine appearances, including an appearance in January 1987 where he was shown a red card just 85 seconds into a game against Southampton. The sending-off is the quickest dismissal in club history. The majority of his appearances in the 1987–88 season were as a substitute, and, frustrated by lack of first-team opportunities, he joined Newcastle United in a £275,000 move in November 1988, after 36 appearances (31 in the League) & two goals for Manchester United.