They told fans that they never expected to completely fill the huge venue, and were delighted to be back on stage with the full line-up, reported Daily Star.
The superstar boyband put on an energetic show at the huge venue as part if their worldwide 'In a World Like This' tour.
Addressing the fans, Kevin Richardson said, "Two years ago A J McLean mentioned he would rejoin the band and two years later I can't believe we have ... Sold out the O2.
The group are on the road celebrating their 20th anniversary, and are happy to be back with their original line-up - completed by Howie Dorough, Nick Carter and Brian Littrell - with Kevin returning to the band two years ago following his departure in 2006.
Also Read
The 'As Long As You Love Me' hitmakers didn't disappoint, putting on a great show with a hit-filled set including the likes of 'I'll Never Break Your Heart' and 'All I Have To Give'.
The concert also had its share of emotional moments, with the group paying tribute to their families and friends.