Ulster are currently third as the battle for the GUINNESS PRO12 play-offs reaches fever pitch.
And O'Brien says his own personal battle with Ulster's Chris Henry - and the Ulster pack in general - is a decisive area.
"The breakdown is massive, it's a big focus for players and Ulster throw a lot of numbers into the ruck, the likes of Chris Henry and Rory Best," he told the Irish Times.
"They cause havoc by staying in there and we'll be looking to get them out of there as quickly as possible.
"Chris is one of those players who will be involved in everything. He's good around the tackle area, he stays in the middle of the ruck the whole time.
"He's a massive threat. It's great to see him back and it's another option for Ireland leading into a World Cup year. I'd say Ulster are delighted to have him too."
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