He signed his professional forms at 17 and had his professional debut in 1999 against Aston Villa at Villa Park in a 3-0 Football League Cup defeat.
Following loan spells at Bournemouth and Royal Antwerp he returned to Manchester and began to feature in the United first team in the 2002-03 season, demonstrating versatility by playing at left-back, right-back, centre-back and central midfield during the successful 2002-03 Premiership campaign.
In 2003-04, United were without Rio Ferdinand after he began a suspension for missing a drugs test in January, and O'Shea took over from Ferdinand in central defence, helping United reach the final of the FA Cup where they triumphed 3-0 over Millwall.
O'Shea earned his first senior cap for the Republic of Ireland on 15 August 2001, when he came on as an 84th minute substitute at home to Croatia. His debut ended in disappointment, however, when he gave away a penalty in injury time, which resulted in Ireland conceding their lead, and the game ending 2-2.
He displayed indifferent form in the 2004-05 season and was linked with a move away from Manchester, with Newcastle and Liverpool being linked with the Irishman. One of the highlights of Manchester United's otherwise slightly disappointing season was the 4-2 away victory against Arsenal, in which O'Shea scored the fourth goal by chipping Arsenal goalkeeper Manuel Almunia from the edge of the 18 yard box.
An injury to Gary Neville in the 2005-06 season also gave O'Shea more first-team opportunities. He was criticised for his lacklustre performances that season, and was reportedly one of the players lambasted by United veteran Roy Keane in a controversial interview on the club's MUTV channel.
On 4 February 2007, during league game against Tottenham Hotspur, O'Shea deputised for Edwin van der Sar in goal after the van der Sar was taken off the pitch for a broken nose, while Manchester United had already used all three substitutes. During this time he denied his Republic of Ireland teammate Robbie Keane a goal with a save a few minutes before full time. Following this incident, United fans chanted "Ireland's number one" in O'Shea's honour.
A month later he won over many United fans by scoring an injury time winner against Liverpool at Anfield for United in the league, having come on as a substitute for Wayne Rooney. This goal was important in United going on to regain the Premiership trophy for the 2006-07 season. He also rescued his team against Everton in one of the final games of the season, when he bundled the ball in the net after Everton goalkeeper Iain Turner fumbled a Ryan Giggs corner. United went on to win 4-2. This was perhaps an even more decisive goal, as Bolton were drawing with Chelsea at the same time and defeat would have let Chelsea back into the title race. He scored another goal from close range, in a 2-1 defeat away to Portsmouth that season, this goal came during a injury hit part of the season, were O'Shea, as a versatile player, was required to play at full back.
John O'Shea had a shooting accuracy of 100% and scored with 80% of his shots in the 2006-2007 season.
During the 2007-08 season, Manchester United have used O'Shea as an emergency striker due to injury problems. His use as a striker gives him the distinction of having played in every position for Manchester United.
On 23 November 2007, John O'Shea extended his contract at Manchester United, to keep him at the club until 2012.
Throughout the 2007-2008 season John O'Shea came off the bench several times and proved important as a utility player on the way to a European double. O'Shea captained his club for the first time, during a 2-0 home defeat to Coventry City in the League Cup 4th round.
John has been capped at all levels from Under 15 to Senior.
John can be seen as part of Vincent Butler's Under 15 team in 1997 by clicking here .
He was part of Brian Kerr's Under 16 Championship Winning team.
John made his Senior International debut against Croatia on 15 August 2001 coming on as a sub for Gary Kelly.
His full international start was against Greece in 2002 when Don Givens took over as temporary manager of the Irish team following Mick NMc Carthy's resignation.