Northampton Saints' Phil Dowson is in third place in the top 10 forwards, and the back rower will be looking to inspire his side to victory over Perpignan in the second Heineken Cup semi-final at stadium:mk next Sunday.
O'Brien's Leinster team-mate Jonathan Sexton, meanwhile, is out in front in the list of top 10 backs thanks to his impressive all-round displays, while his direct opponent in the semi-final, Toulouse's David Skrela, is in second place.
In advance of Munster's home Amlin Challenge Cup semi-final against Harlequins on Saturday, Denis Leamy and Doug Howlett are highly ranked in the respective forwards and backs lists, while Quins playmaker Nick Evans is in seventh place in the Index of leading backs.
To view the Amlin Opta Index preview for the semi-finals, please click here.
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