The Edmonton Eskimos picked O'Shea in the 1st round (4th overall) in the 1993 CFL Canadian College Draft. However, he was soon after traded to Hamilton by Edmonton along with QB DeChane Cameron, linebacker DeWayne Odom and a negotiation list player in exchange for QB Damon Allen on February 16, 1993.
In his first season in the CFL in 1993, O'Shea started in all 18 games. In 1994, he once again played in all 18 games during the regular season. He had 95 DT, 11 special teams tackles and 5 interceptions during the regular season. He was also named to the East Division All-Star team at middle linebacker. 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. He was named to the North Division All-Star team. In Hamilton’s 31–13 loss to the Calgary Stampeders in the North Division Semi-Final, made 4 DT and 1 special teams tackle.
Toronto Argonauts (1996-1999)
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. He played in the last 8 games of the regular season started Toronto’s final 5 games of the regular season. He helped lead Toronto to the 1996 Grey Cup game in a win over Edmonton. He would also help Toronto win its second Grey Cup in as many years in the 1997 game. O'Shea would also play in Toronto in the 1998 and 1999 season as middle linebacker, putting up good numbers.
Second Stint with the Hamilton Tiger-Cats (2000)
Hamilton acquired O'Shea in a trade with Toronto in exchange for RB Éric Lapointe, the right of first refusal on all offensive lineman and the playing rights to Orlondo Steinauer on June 11, 2000. He made an immediate impact in the season opener vs. B.C. as he contributed with 7 DT, made 3 DT On October 13, 2000 in the Tiger-Cats’ 32–8 loss to Toronto, O'Shea had his best individual game of the season with 12 DT and 1 pass knockdown. He finished the season third in the league with 90 DT.
Second Stint with the Toronto Argonauts (2001-2008)
O'Shea signed with the Argonauts once again as a free agent in the 2001 season. The Argonauts had a horrible 2001 season. In 2002, the Argos seemed to turn it around, and made it to the East final only to lose to Montreal. In his second tenure with the Argonauts, O'Shea was an important component of the Argonauts' strong defence. In 2004, O'Shea helped lead the Argos to the 2004 Grey Cup Championship, beating the B.C. Lions 27–19, winning his 3rd Grey Cup, all with the Argonauts.
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 2007, he was voted as the middle linebacker on the All-Time Argos Team.[1] O'Shea was released by the Argonauts on February 27, 2009. He finished his career with the second most games played by an Argonaut with 205, 17 games behind former linebacker Don Moen.
The Winnipeg Blue Bombers hired O'Shea on December 4, 2013, he became the team's 30th Head Coach in team history. In his sixth year with the team, O'Shea won his first Grey Cup as a head coach, defeating the Hamilton Tiger-Cats and ending a 29-year drought in Winnipeg with a win in the 107th Grey Cup game.[3] O'Shea had helped establish a winning foundation for the Bombers, following four successful double-digit win seasons. As a result of this, the Blue Bombers extended him for an additional three seasons through to 2022.[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. He was named coach of the year in both seasons.[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.[7]
O'Shea and his wife have three children; one son and two daughters. In August 2022 the City of North Bay, Ontario named a football field at the Steve Omischl Sports Complex after him.[8]