A calm and collected central defender, O'Leary was noted for his good positioning and elegant style of play. He won his first major honour with Arsenal when he played in their 3–2 win over Manchester United in the 1979 FA Cup Final. He also played in the 1978 and 1980 Cup finals, and the 1980 Cup Winners' Cup final, all of which Arsenal lost. In 1982, O'Leary became club captain, but relinquished it to Graham Rix 18 months later.