O'Donovan played schoolboy football for Leeds A.F.C. Cork. In 2001 he signed for English Premier League club Coventry City as a trainee, before being released in December 2004 at aged nineteen without making a first team appearance for the club.
O'Donovan moved back to Republic of Ireland and joined his hometown club Cork City in early 2005. In his first season at Cork City they captured the League of Ireland championship after a twelve-year absence. He played mainly as a wide-right attacking midfielder after joining Cork, but was soon moved up front as a striker, becoming the top goalscorer in the League of Ireland. He scored in a UEFA Cup win in July 2005 versus Ekranas of Lithuania, City were eventually eliminated by Slavia Prague . The following season they competed in the Champions league qualifying phase but came up short against Red Star Belgrade (Crevena Zvezda). He was named the eircom/Soccer Writers Association of Ireland (SWAI) Player of the Month for November 2005 and won the award again in November 2006 after being switched to centre forward from midfield, he was also a nominee for the Soccer Writers Association of Ireland (SWAI) player of the year the same season.
On 21 November it was reported that Wolverhampton Wanderers were interested in signing O'Donovan, but that they were denied permission to speak to him about a move by Cork City. On 7 July 2007, he scored as Cork City drew 1–1 with Swedish Allsvenskan club Hammarby in the second round of the 2007 Intertoto Cup. He scored a total of 31 league goals in 74 league appearances for the club was top scorer in the 2007 Setanta Cup, as well as competing in the qualifying rounds of the Champions League in 2006 scoring 2 goals in European competitions in his time at CCFC.
In July 2007, Fulham agreed a fee with Cork City to sign O'Donovan, however, Roy Keane signed him for Sunderland on 7 August for a League of Ireland record transfer fee of €500,000, with O'Donovan saying, "I had a choice between Fulham and Sunderland but when I spoke to the gaffer [Roy Keane] my mind was made up." The fee can increase to €1 million depending on appearances for club and country. He signed a three-year contract, He made his Premier League debut for Sunderland as a 70th-minute substitute in the 2–2 draw with Birmingham City at St Andrew's on 15 August. His first start of the season came on 21 October in the 3–1 defeat to West Ham United at Upton Park, although he was substituted in the 46th minute. O'Donovan made seventeen appearances for the Black Cats in the 2007–08 season, starting just four Barclays Premier League games.
His first goal for Sunderland came in a pre-season against Portuguese Liga club Sporting Clube de Portugal on 20 July 2008. He also scored friendly goals against Cobh Ramblers FC, Darlington FC and Falkirk FC.
On 8 August 2008, he signed a one-year loan deal with Dundee United, under then manager Craig Levein. He made his debut for the Tangerines on 11 August in a 3–1 defeat to Hamilton Academical at New Douglas Park. He scored his first goal in a 2–0 home win over St Mirren on 25 October, after coming on as a 67th-minute substitute: the goal was a poignant one, coming in front of a full stadium of supporters who had turned up to commemorate the passing of the popular club chairman Eddie Thompson. On 1 November, he was sent off in the 57th minute of the 0–0 draw against Falkirk at the Falkirk Stadium. In late December he was one of three players along with Michael Chopra and Anthony Stokes recalled by Sunderland who were struggling with injuries.
On 9 January 2009, he signed for Championship club Blackpool on loan until the end of the season. He made his debut for the Seasiders in the 2–1 defeat to Coventry City at the Ricoh Arena on 17 January. He played mainly as a right sided midfielder and wore the famous number 7 shirt. What had been a successful loan spell up to this point took a turn for the worse when on 10 March, he was taken to hospital after being taken ill with suspected appendicitis before Blackpool's game against Sheffield United at Bramall Lane. Two days later he had keyhole surgery to remove his appendix. He returned to action just a month later on 11 April in Blackpool's 1–0 West Lancashire Derby win over Preston North End at Deepdale He retained his place in the side for the following game, a 2–2 draw against Reading.
On 16 September 2009, O'Donovan joined Southend United on a one-month emergency loan deal. He scored on his debut against Brighton & Hove Albion, in the 3–2 win. O'Donovan made a further two league appearances plus one in the Football League Trophy before returning to Sunderland once the loan was completed.
On 23 February 2010, he joined Hartlepool United on loan until the end of the 2009–10 season. He made his debut for Hartlepool in their 4–1 home win over Carlisle United on 23 February 2010. On 6 March 2010, he scored his first goals for Hartlepool, a hat-trick against Southend, who he was with at the beginning of the season. O'Donovan went on to make 15 appearance scoring 9 goals before returning to parent club Sunderland at the end of the season.
O'Donovan signed a three–year deal with Coventry City on 25 June 2010 becoming new manager Aidy Boothroyd's third signing of the close season. He officially transferred to Coventry on 1 July 2010, five years after leaving them as an academy player. He scored his first goal for the Sky Blues against Bury FC in August 2011. O'Donovan suffered a fractured metatarsal in an nPower championship game against Barnsley in November 2011 which meant a significant period recovering on the sidelines. He signed on loan for Hibernian FC on his return to fitness on January 31, 2012 for the remainder of the season. O'Donovan returned to Coventry for the 2012–13 season and made some first team appearances under the new management of Mark Robins, but mutually agreed to terminate his contract on 31 January 2013.
O'Donovan was loaned to Hibernian in January 2012 for the rest of the 2011–12 season where he scored two goals against Kilmarnock FC and a goal in the Scottish cup quarter final against Ayr United en route to the Scottish Cup Final at Hamden Park that season, O'Donovans goal versus Kilmarnock FC was a memorable one and an SPL goal of the month nominee>
O'Donovan signed for Northampton Town on a free transfer in January 2013. He was signed by Aidy Boothroyd, who had previously signed O'Donovan for Coventry. On 13 May 2013 he played in the 3-0 play-off final defeat to Bradford City FC at Wembley Stadium, he signed a new two-year contract in July 2013 at sixfields and started the season in goal scoring form but injuries curtailed his involvement in the first team after double hernia surgery in October led to a ten-week hiatus. In January 2014 following the dismissal of manager Aidy Boothroyd, O'Donovan flew to Dubai for talks with Brunei DPMM FC at the invitation of the Crown Prince of Brunei. A move he later turned down.
After previously rejecting a contract after initial talks in January, O'Donovan signed a contract with Brunei DPMM FC in February 2014 as their marquee player in the Singapore S.League. DPMM are currently managed by former Blackburn Rovers FC manager Steve Kean. He won his first pieces of silverware in Asia when picking up the Starhub S.League Cup in July 2014 and was also voted as the Most Valuable Player of the tournament. O'Donovan finished the season with 26 goals in total and as a firm fans favourite.
In February 2015, he was signed by Mitra Kukar of the Indonesian Super League. Having enjoyed success in Asia the previous season scoring 26 goals in 35 games in the Singapore S.League in all competitions with Brunei DPMM. He was to spend only two months in Indonesia due the newly formed Indonesian Super League (ISL) being scrapped by FIFA regarding ongoing issues with the football association of Indonesia.
On 30 May 2015 the Central Coast Mariners announced that they had signed O'Donovan to a 2-season contract. He made his competitive debut for the club in an FFA Cup match against Wellington Phoenix !Wellington Phoenix on 12 August 2015. He missed a penalty late in the same match. In his A-League debut against Perth Glory !Perth Glory on 10 October 2015, O'Donovan opened the scoring in the game finding the net after a shot from Fábio Ferreira was rebounded to him by the glory keeper Ante Covic. The Mariners won the match 3–2.
On 31 December 2015 O'Donovan was involved in an incident in a match versus the Wellington Phoenix, where he deliberately head-butted Phoenix defender Manny Muscat in retaliation for an off the ball elbow he received from the defender, O'Donovan was sanctioned during the match for the act as the referee awarded the player a yellow card. However, he was later handed an eight-match ban by the Football Federation Australia disciplinary committee for his conduct, one of the longest bans in A-League history. Manny Muscat also received a multiple game suspension for his elbow that triggered the incident. O'Donovan finished his first season in Australia as the clubs golden boot winner as well as the winner of the Hyundai A League 'Goal of the Season'
In April 2008 O'Donovan was named in his first senior squad by new manager Giovanni Trapattoni for the upcoming friendly games against Serbia and Colombia. O'Donovan has played for the Republic of Ireland Under-19, Under-21 and B teams. He made his debut for the Under-21 team in August 2005 against Northern Ireland. On 18 May, he scored as the Irish Under-21's beat Azerbaijan 3–0 in a 2007 UEFA European Under-21 Football Championship qualification preliminary round match, He went on to make eight appearances for the under-21's, scoring once.
On 21 November 2007, he played for Republic of Ireland B as they drew 1–1 with Scotland B in a friendly international at the Excelsior Stadium, Airdrie.
In May 2007, O'Donovan expressed his surprise and disappointment at not being part of the Republic of Ireland squad for the end of season international matches against Ecuador and Bolivia, a squad in which his Cork City teammate Joe Gamble was included. O'Donovan said, "I’m surprised to be honest with you. I’ve scored 25 goals in the last 25 or 26 games so I can’t do any more. I would have liked a reason why I’m not in it but that's life."As of 23 December 2014
Eircom League Premier League Champion 2005
Starhub Singapore League Cup Champion 2014
Scottish Cup Final Runner Up 2013
A-League Goal of the Year: 2015–16
Starhub S-League League Cup Most Valuable Player 2014