O'Dea has played as a centre back for clubs in Scotland, England, Canada, Ukraine and India, and represented the Republic of Ireland internationally.

[1] After making his way through the reserves and academy side, he made his first-team debut for Celtic in the Scottish League Cup against St Mirren in September 2006.

[2] A month later, O'Dea made his league debut against Inverness Caledonian Thistle, where he played the last thirty minutes after substituting for Gary Caldwell.

[5] After playing 16 times for Celtic, O'Dea signed a three-year contract in March 2007 and considered it a "reward for the hard work" he had done.

O'Dea was appointed stand-in captain after Stephen McManus went on loan to Middlesbrough and Gary Caldwell left for Wigan.

[19] In January O'Dea said he was uncertain if his loan would be extended because of the sacking of Ipswich manager Roy Keane, expressing his desire to stay.

[21] Vital Football praised O'Dea's performance in a league cup match against Arsenal, with Jewell calling it "tremendous and crucial in the result".

[25] The following month Leeds United signed him on loan until the end of the season, and he said his objective on his new club was to win promotion to Premier League.

[30] Against Cardiff City, according to the BBC Leeds conceded a goal due to O'Dea's "indecision" which "opened the door for Joe Mason".

[31] In a January 2012 interview he expressed his desire to sign permanently with Leeds,[32] although he received a red card in a loss to Blackpool.

[36] After spending the previous two seasons out on loan, O'Dea was released by Celtic at the end of his contract on 1 June 2012 despite a one-year contract-extension option.

[40] Manager Paul Mariner named O'Dea team captain in September, succeeding the injured Torsten Frings.

[47] Originally part of a transfer (a "win-win for both parties", according to head coach Ryan Nelsen, since O'Dea was the highest-paid player in the league), he was released by Toronto on 18 July 2013 to sign with FC Metalurh Donetsk.

O'Dea played a total of 26 times for Toronto, scoring one goal and setting up three,[49] and was succeeded as team captain by Steven Caldwell.

[64] In June 2017, O'Dea was named as permanent club captain by Dundee, having performed the role during the previous season in place of the injured James McPake.

[66][67][68] O'Dea signed for his local side East Kilbride in May 2019 but left in August without making a single Lowland league appearance for the club.

Less than a week later, O'Dea was named to the international squad for Ireland's UEFA Euro 2008 qualifying Group D matches on 24 and 28 March.

[73][74] On 13 May 2008, he was listed on Giovanni Trapattoni's first squad as Ireland's manager for the 2010 FIFA World Cup qualification – UEFA Group 8 matches against Bulgaria and Italy.

[75] O'Dea made his senior-team debut on 8 September 2009 in a 1–0 friendly win over South Africa at Thomond Park in Limerick as a 60th-minute substitute for Paul McShane.