In 1995, O'Shea moved to England so that he could play in a top division tournament, where he represented London Irish. He helped London Irish to promotion to the top division in England in his first season at the club, and helped his side to the semi-finals of the 1995–96 Pilkington Cup, losing to Leicester 46–21. In London Irish first season in the top flight division, they finished 10th, which meant they had to play in the Relegation/Promotion play–offs against Coventry - London Irish won on aggregate 42–23. In the inaugural European Challenge Cup, London Irish finished bottom of their pool, losing all 5 games. Though in the 1997–98 European Challenge Cup, Irish improved greatly to finish second in their group behind Stade Français. In 1998/99, London Irish recorded their best positioning on the English Premiership table finishing 7th on the overall table. In 1999 he was awarded the Zurich Players’ Player of the Season Award in a season where he captained the side to the top of their European Challenge Cup table, and the semi-finals of the Tetley's Bitter Cup. However, in both competition, London Irish were defeated by Northampton and Castres Olympique respectively.