O'Brien was to the fore as the Lions racked up a dominant 41-16 win and the Leinster flanker revealed that getting the chance to start a Test made the experience all the more special.
"It was brilliant to be part of a winning Test series, and to experience what the Lions is all about. It's a huge part of my life now and I won't forget it for a long time," he told the Irish Examiner.
"Being involved in the Test team makes a difference to the experience. It was very frustrating not being included in the first two, but starting and winning that final game, and playing as well as we did, was very pleasing.
"You are of course fighting for positions with the guys around you and that competitive nature was evident throughout the tour. But the camaraderie was there too, and we enjoyed each other's company."