O'Driscoll made his debut for Leinster in 1999, and under head coach Matt Williams and backs coach Alan Gaffney he became an explosive force in the Leinster backline, forming a highly effective centre partnership with Shane Horgan.
In 2001, Leinster won the inaugural Celtic League beating Munster in Lansdowne Road.
In 2003, Leinster were heavy favourites for that year's Heineken Cup but were beaten by Perpignan in the semi-finals.

Leinster appointed Michael Cheika in the summer of 2005 and despite rumours of O'Driscoll moving to France, O'Driscoll agreed to another year in Ireland.
That year, O'Driscoll returning from a shoulder injury suffered on the Lions tour, would assume the captaincy for the season.
Under backs coach David Knox and the movement of Argentine international Felipe Contepomi to fly half, the Leinster back line became one of the most potent in Europe.

O'Driscoll excelled in wins over Bath Rugby and Toulouse away from home.
These victories would set up a Heineken Cup semi-final in Lansdowne Road against Munster, but defeat against the eventual champions Munster would deny O'Driscoll and his team a final berth.
Leinster were also denied a Magners League title, with David Humphreys of Ulster slotting an injury time drop goal to give victory to Ulster in their final game of the season.

In 2007, Wasps beat Leinster in the quarter-finals of the Heineken Cup.
In 2008, Leinster lost in the group stages.
In 2007, Leinster reached the final hurdle of the Magners league only to be denied by the Ospreys and Cardiff.
In 2008, Leinster won that title ahead of Munster, marking O'Driscoll's second honour with the province, his first and only as captain.

The 2008–09 season marked a shift in focus for O'Driscoll.
While retaining the Irish captaincy under new coach Declan Kidney, he handed the honour of Leinster captain to Leo Cullen.
O'Driscoll scored two tries in the defeat of English champions London Wasps, however this victory was followed by away defeats for Leinster to both London Wasps and Castres.
Leinster advanced to the quarter-finals to face Harlequins at the Twickenham Stoop.
When O'Driscoll was absent through injury in April 2009, Leinster relinquished their Magners League crown to Munster at Thomond Park.
Against Harlequins, Leinster scored a 6–5 victory, the game infamous for the Bloodgate incident.

In the semi-final against rivals Munster in Croke Park, an 82,206 sell out, O'Driscoll was awarded the man of the match award (which he later dedicated to the entire team) after an intercept try completed a convincing win for Leinster and sent O'Driscoll to his first Heineken Cup final.
Leinster captured the Heineken Cup against Leicester Tigers on 23 May 2009.
The 19–16 Heineken Cup victory included a drop goal from O'Driscoll who was suffering from a shoulder injury.

In the 2010–11 season, O'Driscoll won his second Heineken Cup with Leinster.
Having suffered a knee injury in a Magners League game one week before the final, O'Driscoll was a major doubt, but he recovered sufficiently and was named in the starting XV.
In the final held at the Millennium Stadium, Cardiff on 21 May 2011, Leinster came from behind to defeat Northampton 33–22.

In the 2012 Heineken Cup Final O'Driscoll won his third Heineken cup with Leinster.
They beat Ulster 42–14 in an entertaining battle.
Seán O'Brien opened the scoring for Leinster to put them ahead 7–3.
At half time the score was 14–6, with Cian Healy scoring the second try for Leinster but there was still a lot to play for.
A penalty try was then awarded for Leinster but Ulster pulled a try back from second-row Dan Tuohy.
Heinke van der Merwe then settled the game with a try for Leinster and Seán Cronin made sure of Leinster's success.

O'Driscoll said in September 2013, that the 2013–14 season would be his last before retiring.

His final appearance for Leinster, and for any team, was in the 2014 Pro12 Grand Final on 31 May in Leinster's home ground, the RDS in Dublin, in which Leinster defeated the Glasgow Warriors 34–12.
He left the field injured early in the first half to be
O'Driscoll made his debut for Leinster in 1999, and under head coach Matt Williams and backs coach Alan Gaffney he became an explosive force in the Leinster backline, forming a highly effective centre partnership with Shane Horgan. In 2001, Leinster won the inaugural Celtic League beating Munster in Lansdowne Road. In 2003, Leinster were heavy favourites for that year's Heineken Cup but were beaten by Perpignan in the semi-finals.
Leinster appointed Michael Cheika in the summer of 2005 and despite rumours of O'Driscoll moving to France, O'Driscoll agreed to another year in Ireland. That year, O'Driscoll returning from a shoulder injury suffered on the Lions tour, would assume the captaincy for the season. Under backs coach David Knox and the movement of Argentine international Felipe Contepomi to fly half, the Leinster back line became one of the most potent in Europe.
O'Driscoll excelled in wins over Bath Rugby and Toulouse away from home. These victories would set up a Heineken Cup semi-final in Lansdowne Road against Munster, but defeat against the eventual champions Munster would deny O'Driscoll and his team a final berth. Leinster were also denied a Magners League title, with David Humphreys of Ulster slotting an injury time drop goal to give victory to Ulster in their final game of the season.
In 2007, Wasps beat Leinster in the quarter-finals of the Heineken Cup. In 2008, Leinster lost in the group stages. In 2007, Leinster reached the final hurdle of the Magners league only to be denied by the Ospreys and Cardiff. In 2008, Leinster won that title ahead of Munster, marking O'Driscoll's second honour with the province, his first and only as captain.
The 2008–09 season marked a shift in focus for O'Driscoll. While retaining the Irish captaincy under new coach Declan Kidney, he handed the honour of Leinster captain to Leo Cullen. O'Driscoll scored two tries in the defeat of English champions London Wasps, however this victory was followed by away defeats for Leinster to both London Wasps and Castres. Leinster advanced to the quarter-finals to face Harlequins at the Twickenham Stoop. When O'Driscoll was absent through injury in April 2009, Leinster relinquished their Magners League crown to Munster at Thomond Park. Against Harlequins, Leinster scored a 6–5 victory, the game infamous for the Bloodgate incident.
In the semi-final against rivals Munster in Croke Park, an 82,206 sell out, O'Driscoll was awarded the man of the match award (which he later dedicated to the entire team) after an intercept try completed a convincing win for Leinster and sent O'Driscoll to his first Heineken Cup final. Leinster captured the Heineken Cup against Leicester Tigers on 23 May 2009. The 19–16 Heineken Cup victory included a drop goal from O'Driscoll who was suffering from a shoulder injury.
In the 2010–11 season, O'Driscoll won his second Heineken Cup with Leinster. Having suffered a knee injury in a Magners League game one week before the final, O'Driscoll was a major doubt, but he recovered sufficiently and was named in the starting XV. In the final held at the Millennium Stadium, Cardiff on 21 May 2011, Leinster came from behind to defeat Northampton 33–22.
In the 2012 Heineken Cup Final O'Driscoll won his third Heineken cup with Leinster. They beat Ulster 42–14 in an entertaining battle. Seán O'Brien opened the scoring for Leinster to put them ahead 7–3. At half time the score was 14–6, with Cian Healy scoring the second try for Leinster but there was still a lot to play for. A penalty try was then awarded for Leinster but Ulster pulled a try back from second-row Dan Tuohy. Heinke van der Merwe then settled the game with a try for Leinster and Seán Cronin made sure of Leinster's success.
O'Driscoll said in September 2013, that the 2013–14 season would be his last before retiring.
His final appearance for Leinster, and for any team, was in the 2014 Pro12 Grand Final on 31 May in Leinster's home ground, the RDS in Dublin, in which Leinster defeated the Glasgow Warriors 34–12. He left the field injured early in the first half to be 
Brian O'Driscoll