In 2002, O'Shea made his debut at the BDO World Championship, narrowly losing to former Masters champion Colin Monk 3–2.

[1] In the 2009 BDO World Championship, O'Shea defeated Anderson in the quarter-finals, his best friend Darryl Fitton in the semi-final, but was once again second-best in a major final as he lost to Hankey 7–6.

[1] O'Shea entered the 2009 Winmau World Masters as the number one seed but despite 3–0 wins over Dave Prins and eighth seed Joey ten Berge, he lost in the semi-finals to defending champion Martin Adams who successfully retained his title.

O'Shea then won the WDF World Cup men's singles title with victory over ten Berge in the final.

[1] O'Shea won all three group games in the 2010 Grand Slam of Darts, defeating Vincent van der Voort 5–1, James Wade 5–1 and Justin Pipe 5–4.

[2] In October 2012, O'Shea reached his sixth major final at the 2012 Winmau World Masters, and was beaten by Stephen Bunting.