The Lions head to New Zealand this summer
Former Scotland and Lions flanker Finlay Calder believes who the tourists choose at openside flanker could decide who wins the test series.
Calder led the Lions to a series win in Australia in 1989 and thinks O'Briens presence could be the difference between the teams.
Speaking to Scottish rugby's The Scrum magazine he said: "For me, if they get seven right they have a chance, and for me that man should unquestionably be Sean O'Brien, who is the best in Europe if not the world.
"It's tricky because Warren Gatland will have Sam Warburton in his mind as a potential captain, but nobody comes close to O'Brien if he's fit.
"You can't have your captain sweating over whether or not he's going to be selected, so I'd consider someone other than Warburton, who is in his own right a fine player," Calder said.
"I just think O'Brien will be the last guy the All Blacks want to come up against; he's be an absolute nightmare to face and could be the difference between winning and losing that tour," he added.
Lions head coach Warren Gatland: ©INPHO/Billy Stickland
The British and Irish Lions head to New Zealand having won their previous test series in Australia 2-1 in 2013. Warren Gatland is due to pick his squad for the tour after this season's 6 Nations Championship.