He is a former Canadian football linebacker and former special teams coordinator of the Toronto Argonauts of the CFL from 2010 to 2013, winning the Grey Cup in 2012.

The Edmonton Eskimos picked O'Shea in the 1st round (4th overall) in the 1993 CFL Canadian College Draft.

In 1995, O'Shea played in 10 games for Hamilton, made 69 DT, 4 special teams tackles and intercepted 2 passes during the regular season.

In Hamilton's 31–13 loss to the Calgary Stampeders in the North Division Semi-Final, made 4 DT and 1 special teams tackle.

O'Shea signed with the Argonauts half way through the 1996 season after he was one of the final cuts in the Detroit Lions training camp.

O'Shea would also play in Toronto in the 1998 and 1999 season as middle linebacker, putting up good numbers.

Against the Saskatchewan Roughriders on October 20, 2006, he became the first Canadian and third player in CFL history to record 1000 career tackles.

In 2017, the Blue Bombers hosted their first playoff game in O'Shea's tenure, but lost 39-32 to Edmonton.

In 2018, the team won its first playoff game, 23-18 in the West Semi-Final vs Saskatchewan, and appeared in its first division final.

[4][3] O'Shea would help guide the Bombers to two more Grey Cup appearances, winning in 2021 and losing to the Argonauts to close out the 2022 season.

[5][6] Following three consecutive trips to the championship match O'Shea and the Blue Bombers agreed on a three-year contract extension in December 2022.

In August 2022 the City of North Bay, Ontario named a football field at the Steve Omischl Sports Complex after him.