Having played a game fewer than the other teams in the top four, Leinster are well-placed for a play-off spot, with a crucial game at the in-form Ospreys next up.
O'Brien, their influential flanker, has been mentioned as a potential Ireland captain in the Six Nations, with Joe Schmidt looking to replace Munster legend Paul O'Connell.
And for former Leinster skipper Cullen, O'Brien has exactly what is required to take on a leadership role, whether he has the official title of captain or not.
"[O'Brien] shows a lot of leadership qualities for sure," said Cullen.
"He plays a big role in our leadership group so I'm sure he will play a big role with Ireland's leadership group whether he is actually captain or not. He is definitely a strong leadership figure for Leinster or Ireland."
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